Handling the risks of legacy system decommissioning in Public Sector projects

Project overview

Legacy systems are a persistent challenge across the public sector. Built to meet the needs of a different era, many continue to underpin critical services long after their original lifespan. While they often remain operational, they create growing risks in terms of resilience, security, data integrity and cost.

Our consultancy was commissioned by a central government organisation responsible for administering a high-volume, citizen-facing service. The service relied on a legacy platform over 20 years old. The system had been patched and extended over time, but vendor support was ending, integration points were fragile and the underlying infrastructure presented increasing security and operational risks.

The organisation had already procured a modern, cloud-based replacement platform. The challenge was not simply to introduce new technology, but to safely retire the legacy system without disrupting service delivery, losing critical data or exposing the organisation to compliance risks.

Our engagement spanned nine months, covering discovery, transition planning and phased decommissioning. We worked alongside internal digital, security, operational and commercial teams to design and implement an end-to-end transition approach. Our role was to bring structured delivery, service design and business analysis capability to a complex and sensitive change.

The objective was clear: decommission the legacy platform in a controlled, secure and user-centred way, while maintaining service continuity for citizens and operational teams.

Problem

At first glance, the legacy system appeared stable. It processed thousands of transactions each month and remained embedded in operational workflows. However, beneath the surface, the risks were compounding.

The system stored over a decade of citizen records in a proprietary format. Data structures had evolved without consistent governance. Interfaces with downstream systems were undocumented or partially understood. Manual workarounds had grown around system limitations, creating hidden dependencies across teams.

From a service perspective, the risks were significant. Any disruption could delay payments, prevent case progression or erode trust among citizens who relied on the service. Internally, staff were concerned about losing access to historical case data needed for audit, appeals or safeguarding.

Security was an increasing concern. The infrastructure sat outside modern security monitoring standards, and patching required specialist support that was becoming harder to source. The organisation’s security team had flagged the system as a material risk in internal audits.

There was also a strategic imperative. The continued operation of the legacy platform constrained future service improvements. Integration with new digital services was complex and costly. Maintaining dual systems indefinitely would undermine the return on investment in the new platform.

The organisation faced a delicate balancing act. They needed to move decisively to reduce risk, but any misstep could affect vulnerable users and attract public scrutiny. The problem was not simply technical. It was about safeguarding public value while modernising critical infrastructure.

“What stood out was their transparency and willingness to upskill our internal teams. From data mapping workshops to security reviews, they made sure we understood the ‘why’ behind every decision, not just the outcome.”

Lead Technical Architect

Research and Discovery

We began with a structured discovery phase focused on understanding the service end to end. This included stakeholder interviews, service mapping workshops and targeted analysis of data and infrastructure.

Our first step was to establish a shared understanding of the current state. Working with operational leads, we mapped the lifecycle of a case from submission to closure, identifying where the legacy system was used and how data flowed across teams. This exercise surfaced undocumented dependencies, including manual reconciliation processes and locally stored data extracts.

In parallel, we worked with technical architects and security specialists to document system interfaces, hosting arrangements and data classifications. This revealed several integration points that were business-critical but poorly documented. Some batch processes had no clear owner.

We conducted workshops with frontline staff to understand how the legacy system supported day-to-day work. These sessions highlighted practical concerns about losing search functionality, access to historical notes and reporting capabilities. Staff were not resistant to change, but they were concerned about operational impact during transition.

Data discovery was a critical strand of work. We partnered with data analysts to profile the legacy database, identifying duplicate records, incomplete fields and inconsistent identifiers. This shaped the migration strategy and helped us quantify the scale of cleansing required.

Security and compliance teams were engaged early. Together, we clarified retention requirements, audit obligations and the legal basis for storing historical data. This informed decisions about what needed to be migrated, what could be archived and what could be securely disposed of.

The discovery phase culminated in a set of clearly articulated risks and transition principles. These included maintaining uninterrupted access to active cases, ensuring auditability of migrated records and avoiding a single “big bang” switch-off. The insights gained in discovery shaped a phased and risk-managed approach to decommissioning.

“They took the time to understand not just the system, but how our teams actually used it day to day. The knowledge transfer was structured and practical, which meant we felt confident managing the service long after the legacy platform was switched off.”

Head of Service Operations

Design Approach

Our design approach centred on three core themes: data integrity, service continuity and security by design.

We established a cross-functional transition team comprising business analysts, service designers, technical architects, data specialists and operational representatives. Rather than treating decommissioning as a purely technical exercise, we framed it as a service transformation programme.

We began by defining a target operating model for life after the legacy system. This clarified how cases would be handled, where historical data would reside and how users would access it. By articulating the future state, we were able to design backwards and structure the transition.

Data migration was approached iteratively. We defined data domains and prioritised active and high-risk cases. Data mapping exercises aligned legacy fields to the new platform’s schema, highlighting gaps and transformation rules. We implemented trial migrations in controlled environments to test completeness, accuracy and performance.

Quality assurance was embedded throughout. We defined validation rules and reconciliation processes to ensure migrated data matched source records. Operational staff were involved in user acceptance testing, validating real-world scenarios against live case histories.

To maintain service continuity, we adopted a phased cutover strategy. Instead of switching off the legacy system in one step, we transitioned cohorts of cases in waves. Each wave was preceded by communication, training and contingency planning. During transition periods, dual-running arrangements ensured that new and legacy systems could be cross-checked.

Service design techniques were used to support operational readiness. We developed clear user journeys for staff interacting with the new system and identified points of friction. Where gaps existed, we worked with product teams to refine functionality before migration.

Security considerations were integrated into every stage. Access controls were reviewed and updated in line with modern identity management standards. Data transfers were encrypted and logged. We worked closely with information assurance teams to complete security accreditation activities ahead of each migration wave.

Governance was deliberately lightweight but robust. We established a risk register focused on decommissioning-specific risks and reported regularly to senior stakeholders. Clear decision points were defined, including criteria for progressing from pilot to scaled migration.

Throughout, our consultancy acted as a delivery partner rather than an external observer. We facilitated workshops, documented decisions, aligned teams and translated complex technical issues into accessible language for senior leaders. Our role was to create clarity and momentum in an environment where uncertainty could easily stall progress.

Outcome and Impact

The legacy system was fully decommissioned within the planned timeframe, with no unplanned service outages.

Over 1.2 million historical records were successfully migrated or archived in line with retention policies. Data reconciliation checks demonstrated a 99.98% accuracy rate across migrated fields, exceeding the agreed quality threshold. Active case continuity was maintained, with no missed payments or delayed decisions attributable to the transition.

Operational efficiency improved as a result of consolidating onto a single modern platform. Staff reported a 25% reduction in time spent navigating between systems and manual reconciliation tasks were eliminated. The new platform enabled integrated reporting, reducing monthly performance reporting preparation time by 40%.

From a security perspective, the organisation reduced its exposure to unsupported infrastructure and closed several high-risk audit findings. The decommissioning removed a legacy hosting environment from the network, aligning the service with current security standards and reducing annual infrastructure costs.

Financially, retiring the legacy platform eliminated ongoing licensing and specialist support costs, delivering a projected annual saving of six figures. More importantly, it unlocked the organisation’s ability to iterate and improve the service without being constrained by outdated architecture.

The project also strengthened internal capability. Teams developed clearer data governance practices and improved documentation of interfaces and processes. The structured approach to risk management has since been adopted as a template for other decommissioning initiatives.

Citizens experienced a seamless transition. There were no increases in complaints during migration waves and service performance metrics remained stable throughout.

“The consultancy integrated quickly into our multidisciplinary team. They didn’t operate as an external supplier — they worked alongside us, understood our pressures and helped us navigate complex decisions with clarity and calm.”

Senior Responsible Owner

Reflection

Decommissioning a legacy system is often perceived as a technical milestone. In practice, it is a test of organisational alignment and delivery discipline.

What made this project successful was not simply robust data migration scripts or well-managed infrastructure changes. It was the collective commitment to protecting user outcomes while modernising the underlying service. By framing decommissioning as a service challenge rather than a technical task, we were able to engage operational teams, security specialists and senior leaders in a shared endeavour.

We were reminded that legacy systems persist for reasons beyond technology. They embody years of practice, policy and human workarounds. Retiring them requires respect for that history, alongside clarity about future direction.

For public sector organisations facing similar challenges, early discovery, cross-functional collaboration and phased transition are critical. With the right structure and partnership, legacy risk can be reduced without compromising public trust.

As government continues to modernise, safe and thoughtful decommissioning will remain an essential capability. We are proud to support organisations in navigating that journey and welcome conversations with teams considering their next step.