From Risk to Future-Ready: Rethinking Legacy Migration

Your legacy system isn’t just slowing you down; it’s holding you back from what’s next. For many organisations, legacy technology migration is no longer optional; it is becoming critical to staying operational.

These systems have delivered resilience over many years, but continued modernisation is becoming increasingly important to managing cost and risk in the public sector.

Central governments, local authorities, healthcare and defence—wherever you look, people still rely on vital services running on platforms that never anticipated today’s data needs. Think about SAP stacks from decades ago. SAS 9 systems driving fraud checks and benefits analysis. Bespoke on-premise builds that have evolved over many years, often supported by long-standing, experienced employees.

It is understandable why many organisations choose to keep these systems for now. It is tough to make the budget argument. So another year goes by, often without a clear data migration strategy in place. 

But delaying migration keeps getting more expensive. Technical debt rarely causes immediate disruption until suddenly it does. McKinsey estimates that technical debt can account for 20-40% of the value of an organisation’s entire technology estate before remediation efforts even begin. Addressing the issues proactively can help organisations reduce the risk of disruption and maintain resilience during critical periods. 

So what does “legacy risk” actually look like in everyday life?

Quite often, the challenges associated with legacy infrastructure emerge gradually, rather than a single incident. Analysts may find themselves spending longer than expected reconciling exports because older systems don’t connect smoothly with newer reporting tools. Security teams may need additional steps to complete audits where platforms weren’t originally designed with today’s compliance expectations in mind. And in cloud programmes, progress can sometimes slow when one part of the estate requires extra care to integrate safely with more modern environments.

You also see situations where important operational knowledge sits closely with a small number of people who have worked with these systems for a long time. That creates a natural focus on continuity and knowledge sharing, ensuring that understanding is retained as teams evolve.

These are not unusual or unexpected situations. They are a reflection of how complex, long-lived systems and services evolve over time, particularly in large organisations. And they sit alongside the day-to-day reality of teams working with highly sensitive and important data, doing their best to maintain reliable services while gradually modernising the underlying technology.

A practical way to understand what “legacy risk” looks like at scale is to look at how common it is across the public sector. Recent UK government analysis shows that essential services still run on technology that is decades old, with around 28% of central government systems classified as legacy in 2024, up from 26% the previous year. The picture is uneven across organisations, with legacy systems making up anywhere from 10% to as much as 60–70% in some NHS trusts and police forces. In many cases, organisations also don’t yet have a complete inventory of these systems, which means the full extent of dependency isn’t always clearly visible.

What this means in practice becomes clearer when you look at how some services still operate day to day. In the Home Office, for example, reporting has highlighted that parts of asylum casework continue to rely on systems that have been in place for decades, including a 25-year-old case database, despite most operations having been migrated. In practical terms, that kind of environment means information doesn’t always sit in one place. Instead, it can be distributed across multiple systems that evolved at different times, requiring data to be joined up after the fact rather than flowing through a single, unified record.

Taken together, this doesn’t point to isolated system failure but rather a structural reality that builds up over time. Public services tend to evolve in layers, and the data infrastructure evolves with them. So day-to-day work often involves moving between systems, reconciling records and bridging gaps between older platforms and newer tools. It is less visible than a single outage or incident, but it shapes how work actually gets done behind the scenes, while wider modernisation continues in parallel.

The way we talk about legacy system migration has changed

For years, “legacy migration” was a costly and time-consuming process that rarely achieved its intended goals, which has resulted in some organisations taking a more cautious approach.

But migration isn’t what it used to be. Modern cloud data migration for the government now uses proven frameworks, automation and phased delivery to reduce risk, including iterative and phased migration approaches recommended in UK government guidance. All these factors mean migration can get done—safely, reliably and with less disruption—if you approach it with discipline.

For many organisations, the real question is no longer whether to migrate, but when and how to do it safely.

How we handle legacy migration

Butterfly Data doesn’t believe in forcing everyone into a one-size-fits-all template. Every old data environment is its own puzzle. What we do offer is a clear, repeatable process to cut through uncertainty at each stage, plus deep platform expertise to handle truly complex jobs.

Here is how we break down our approach:

1. Discovery and assessment

We start by mapping your landscape in detail: apps, data stores, dependencies, performance roadblocks, quality issues and technical limits. This gives us the solid foundation we need for any decisions after.

2. Legacy migration strategy and planning

With discovery done, we define a clear data migration strategy: which platform to pick, which order to do things in, how to phase delivery, and how to measure real success, all aimed at minimal business disruption.

3. Data exploration and source-to-target design

We dig into your data, find legacy structures and relationship quirks, and spotlight quality gaps. Then we chart source-to-target mappings with rules and transformations – the blueprint before we write a line of migration code.

4. Data migration development and execution

We create automated workflows for extraction, transformation, filtering and loading, designed to be repeatable, trackable and auditable. Data stays secured through it all.

5. Validation, testing and pilot migration

We run thorough parallel tests to check the data's accuracy, quality and value in the new setting. Pilots reveal real-world impact so we can solve problems before the big cutover.

6. Production migration and transition

We deploy with control, keeping downtime low. And, when it is time, we help retire old systems—scrubbing out technical debt instead of just shifting it around.

7. Post-migration support and optimisation

Right after migration, we stick around to sort early issues, tune performance and make sure everything’s humming in the new environment. We don’t leave until it works, not just when it’s “done.”

Why this matters now

You only have so much time for a controlled, careful migration. As your legacy systems age, the number of individuals who possess in-depth knowledge about them decreases. Security holes grow. And the gap between your systems and what the cloud can do just keeps widening.

HMRC’s upcoming programme to exit legacy data centres and migrate hundreds of critical services to cloud infrastructure reflects a broader government push to reduce technical debt and improve operational resilience.

Organisations that invest in legacy migration and the right foundations gain more than resilience—they unlock scalable data platforms that support everything that comes next, such as advanced analytics, AI and ML, real-time data handling and the kind of decision support today’s services demand.

We are an experienced employee-owned, B-Corp-certified consultancy with a vetted team who are committed to delivering the most effective solutions for our clients. We don’t push unnecessary migrations. We care about making your organisation stronger: technically, operationally and strategically, with the right technology for you.

If you are carrying legacy risk you know you need to fix—or you don’t know where to start—we would love to talk.