Assessing and planning for successful legacy system replacements

Overview

Public sector organisations continue to rely on complex legacy systems that underpin critical services. Many of these platforms were designed decades ago, shaped by policy, technology, and operating models that no longer reflect modern user expectations or contemporary delivery practices. Our consultancy was engaged by a central government organisation responsible for delivering a high‑volume, citizen‑facing service to assess the viability of replacing a core legacy system and to design a realistic, low‑risk modernisation roadmap.

The organisation had reached a clear decision point. The existing system was costly to maintain, difficult to change, and increasingly misaligned with current policy intent and service design ambitions. Previous attempts to modernise had stalled, largely due to the scale of the challenge and uncertainty around risk, cost, and delivery approach. Senior leaders needed a clear, evidence‑based view of the problem space and a pragmatic plan that balanced long‑term transformation with the need to keep critical services running.

Our engagement focused on evaluating the legacy landscape, understanding user and organisational needs, and producing a phased roadmap to support successful system replacement. The work was delivered over a twelve‑week period and combined strategic analysis with hands‑on discovery, working closely with internal digital, policy, and operational teams. While the immediate output was a modernisation plan, the broader aim was to provide confidence, clarity, and momentum for the organisation to move forward.

“From the outset, the consultancy felt like a natural extension of our service team rather than an external supplier. They took the time to understand our constraints and worked alongside us day to day, which made collaboration straightforward and genuinely productive.”

Senior Service Owner

Problem

The legacy system sat at the heart of the organisation’s service delivery model. It supported case management, decision‑making, and a complex set of integrations with internal and external systems. Over time, incremental policy changes and tactical fixes had added layers of complexity, resulting in a tightly coupled platform that was fragile, opaque, and difficult to adapt.

From a user perspective, the system constrained service improvement. Frontline staff relied on manual workarounds to complete routine tasks, increasing processing times and the likelihood of error. Citizens experienced delays and inconsistent outcomes, often driven by system limitations rather than policy intent. These issues directly affected trust in the service and placed avoidable pressure on operational teams.

From an organisational perspective, the system represented a growing risk. Critical knowledge of its inner workings was held by a small number of specialists, some of whom were approaching retirement. Release cycles were slow and expensive, limiting the organisation’s ability to respond to legislative change. There was also an increasing reliance on unsupported technologies, raising concerns around security, resilience, and regulatory compliance.

The challenge was not simply to replace an ageing system, but to do so in a way that protected live services, delivered public value for money, and aligned with government digital and architectural standards. Any solution needed to acknowledge delivery constraints, existing supplier arrangements, and the realities of operating a mission‑critical service while change was underway.

Research and Discovery

Our approach began with a structured discovery phase designed to build a shared understanding of the current state. We worked alongside internal teams to map the service end‑to‑end, combining technical analysis with user‑centred research. This ensured that subsequent decisions were grounded in evidence rather than assumption or anecdote.

We conducted interviews and workshops with a broad range of stakeholders, including operational staff, policy leads, digital teams, and senior sponsors. These conversations surfaced differing perspectives on the system’s purpose and pain points, highlighting where priorities were aligned and where trade‑offs would be required. They also revealed a strong appetite for change, balanced by understandable concerns about delivery risk and service disruption.

In parallel, we supported lightweight user research to understand how the system affected day‑to‑day service delivery. Observations and journey walkthroughs with frontline users exposed the extent of manual intervention required to compensate for system constraints. This evidence helped quantify inefficiencies and provided concrete examples that anchored strategic discussions in real operational experience.

Technical discovery focused on understanding system architecture, data flows, and dependencies. Working closely with internal architects and suppliers, we documented integrations, assessed technical debt, and identified components most suitable for early replacement or decoupling. Findings were captured through clear, accessible artefacts that could be shared beyond the immediate delivery team.

The discovery phase ultimately reframed the problem. Rather than viewing the legacy system as a single monolith to be replaced wholesale, the organisation began to see it as a collection of capabilities with varying levels of risk and value. This shift in perspective became central to shaping a phased, achievable modernisation roadmap.

“They struck the right balance between pace and care. By working closely with our operational and policy colleagues, they helped us make sense of a very complex legacy landscape without overwhelming the team. The approach left us better equipped, not dependent.”

Head of Digital Delivery

Design Approach

Our role was to translate discovery insights into a coherent and actionable modernisation strategy. We applied a service‑led approach throughout, ensuring that technology decisions were driven by user and organisational needs rather than by platform preferences alone.

Working collaboratively, we defined a set of design principles to guide decision‑making. These focused on prioritising user outcomes, reducing coupling between components, enabling incremental delivery, and aligning with established government digital and architectural standards. The principles provided a consistent reference point as options were explored, tested, and evaluated.

Business analysis and service design disciplines were applied in tandem. We supported the organisation to define future‑state service journeys and capability models, describing what the service needed to achieve rather than prescribing how solutions should be built. This helped separate policy intent from technical implementation and created space for more flexible, adaptive delivery approaches.

We facilitated a series of option‑assessment workshops to explore different replacement strategies, ranging from a full system rewrite to incremental component replacement. Each option was assessed against agreed criteria, including cost, delivery timescales, operational risk, and impact on live services. This transparent, evidence‑based approach enabled informed decision‑making and built confidence at senior leadership level.

The resulting roadmap balanced ambition with pragmatism. Early phases focused on stabilisation and risk reduction, such as improving data quality and introducing clearer interfaces between systems. Later phases set out the progressive replacement of high‑value components, allowing benefits to be realised sooner while maintaining service continuity.

Throughout the engagement, we worked as an extension of the client team. Knowledge transfer was embedded in our approach, ensuring that internal staff were equipped to take the work forward. This included co‑creating artefacts, running joint playback sessions, and producing documentation aligned with internal governance and assurance processes.

Outcome and Impact

By the end of the engagement, the organisation had a clear, actionable plan for modernising its legacy system landscape. The roadmap was endorsed by senior stakeholders and used to inform investment decisions and delivery planning for the following financial year.

Key outcomes included a shared understanding of the system’s risks and constraints, articulated in a way that resonated with both technical and non‑technical audiences. The organisation gained clarity on which elements of the system required urgent attention and which could be addressed over a longer timeframe.

The roadmap also identified opportunities to reduce operational overhead by removing manual workarounds and simplifying user journeys. Early modelling suggested the potential to reduce case processing times by up to 20 per cent in initial phases, alongside a measurable reduction in support tickets linked to system errors and avoidable rework.

The engagement strengthened internal capability as well as strategic alignment. Teams reported increased confidence in discussing legacy modernisation with suppliers and improved collaboration between policy, digital, and operational functions. The artefacts produced during discovery and design became durable reference points for subsequent delivery activity.

While full system replacement was beyond the scope of this phase, the organisation moved forward with a clear sense of direction and significantly reduced delivery risk. The work laid strong foundations for incremental change that could be delivered within existing constraints while still supporting long‑term transformation goals.

“What stood out was how deliberately knowledge transfer was built into the work. We weren’t just handed a set of documents at the end — we were involved in shaping them. That meant we finished the engagement with a shared understanding and the confidence to carry the roadmap forward ourselves.”

Lead Technical Architect

Reflection

This project demonstrated the value of treating legacy system replacement as a service transformation challenge rather than a purely technical exercise. By grounding decisions in user needs and organisational context, the organisation was able to move beyond stalled debates and focus on practical, achievable next steps.

The success of the engagement was rooted in partnership. Open collaboration, shared ownership of outcomes, and a willingness to address uncomfortable realities created the conditions for meaningful progress. The work reinforced the importance of discovery as a decision‑making tool, not simply a delivery phase.

As public sector organisations continue to grapple with ageing technology, this project highlights the importance of pragmatic, phased approaches that respect operational realities. Legacy systems will remain a feature of government for the foreseeable future, but with the right methods and mindset, they do not need to be a barrier to delivering better public services.