The practical guide to turning legacy liabilities into digital assets

This article was originally published for techUK as part of their Building Smarter State Week 2025

If you're leading digital transformation in the public sector, you know the challenge: modernising effectively without disrupting essential citizen services or breaking tight budgets. Those legacy applications powering critical operations feel like a double-edged sword - essential yet frustratingly difficult to update or to integrate with newer technologies.

The good news? You don't necessarily need costly wholesale replacement. Many UK public sector organisations are discovering that incremental, modular approaches deliver better outcomes, allowing upgrades based on operational priorities and technical constraints while aligning with GDS principles. 

1. Surface legacy data for modern insights

One of the most cost-effective modernisation approaches is unlocking data trapped in legacy systems. Tools like Power BI and the Power Platform suite enable direct connection to legacy databases for real-time dashboards and new data capture forms.

There are many examples of organisations embracing Power BI. For example, NHS Trusts are creating integrated healthcare analytics by consolidating data from multiple sources to create unified dashboards for real-time patient outcomes. While local authorities are using Power BI to consolidate reporting workloads, drawing data from many disparate legacy systems across their technology estate. 

Key considerations: Care should be taken to ensure that any security and auditing requirements are still honoured when exposing data for reporting through Power BI. Writing new data into legacy databases must be done with safeguards in place to preserve auditing, security, and business rules. 

2. Build API layers for service integration 

For systems with complex business logic that must remain intact, wrapping legacy functionality in an API layer can be transformative and cost-effective. This enables modern interfaces and external systems to interact with legacy applications while preserving proven workflows. 

This strategy is particularly useful in public sector environments, where legacy systems often contain decades of embedded rules and workflows. By exposing functionality through APIs, organisations can build new services and improve user experiences while maintaining the integrity of the original system.

A practical example of this approach is our work with the Isle of Man Ship Registry, where we developed an offline survey application that leveraged APIs to access existing business logic and data layers. The result was a modern, user-friendly interface for ship surveyors that preserved all the established regulatory processes and data integrity of the original system. 

Public sector essentials: Follow government API design standards for future cross-departmental integration, implement approved authentication methods and ensure new interfaces meet WCAG 2.1 AA accessibility standards.

3. Progressive modernisation within spending cycles

Instead of risky big-bang replacements, progressive modernisation involves cataloguing functionality and incrementally replacing high-priority components. This works well within spending review cycles and reduces operational risk.

This approach aligns with Treasury business case requirements - each phase demonstrates clear service improvements and return on investment, making approval processes more manageable.

We took this refactoring approach to help modernise the Crown Commercial Service’s car leasing portal which is used across the whole of the UK’s public sector – preserving core features whilst improving the user interface and delivering new features and functionality.

Implementation framework: Align upgrade phases with budget cycles and maintain parallel running until functional parity has been confirmed.

4. Targeted code conversion for critical components

In extreme cases, especially where legacy code is poorly documented or too complex to reverse engineer, automated code conversion may be the only viable option. Tools like GitHub and Copilot can help translate legacy languages (e.g. COBOL) into modern equivalents like C#.  

We are currently working on a major modernisation project to replace a complex but critical system for the Isle of Man Government. Due to a multitude of factors including the age and underlying technology of the existing system, a rewrite was required. However, for some small sections of existing business logic, the most viable way of accurately recreating the equivalent logic was to use Copilot to directly convert the COBOL to C#.  While this approach carries risk, it can be mitigated using a comprehensive suite of automated “unit” tests to validate functional parity. 

Public sector safeguards: Extensive testing against known scenarios, gradual rollout starting with non-citizen-facing functions, maintained fallback capability, and comprehensive documentation for future teams.

Discovery: The foundation of success

Whatever path you choose, a thorough discovery to address public sector complexities is essential: 

• Technical audit: Map system interfaces, data flows, and integration points 
• Policy and legislative alignment: Review how systems support ministerial priorities and policy objectives 
• User research: Engage staff and citizens to understand pain points 
• Compliance review: Assess security, accessibility, and data protection compliance 
• Stakeholder mapping: Identify affected departments, agencies, and citizen groups 

Key considerations: Consider regulatory or legislative requirements, minimum service levels during transition, staff consultation needs, and applicable procurement frameworks. 

The path forward

Legacy modernisation in the public sector succeeds through incremental, risk-managed approaches that respect unique public service constraints and responsibilities. Whether surfacing data for better decisions, creating APIs for improved citizen interfaces, progressively replacing components, or converting critical code sections, success depends on thorough planning and stakeholder engagement.

The goal isn't just technical modernisation; it's enabling better public services that meet citizen expectations while ensuring responsible stewardship of public resources. Your legacy systems can become the foundation for improved citizen services rather than digital transformation barriers.

With careful planning aligned to government standards and appropriate support, you can balance innovation with stability, reduce risk while delivering real value, and modernise in ways that fit your context while setting up future success.

#LiberateLegacy

To find out more about how you can liberate your legacy systems, visit our Digital Foundations Framework