How organizations decide a system is legacy
The UK guidance turns that definition into a review:
Failing any one of seven criteria is enough to classify a system as legacy.
One criterion is software or hardware past the support date set by its manufacturer or provider.
A system can also count as legacy only because it depends on another system that is itself legacy.
The review has an Action-Required outcome for systems that will become legacy within 12 months without action.
Other organizations draw the line elsewhere. The US Department of Defense treats a system as legacy when it is scheduled for retirement within three years. An earlier GOV.UK discovery found that legacy technology could not be grouped under a single definition.
What keeping a legacy system costs, and the ways out
The cost figures below come from US federal audits, so read them as public-sector data. GAO states that the cost of operating and maintaining legacy systems grows over time.
Of the 11 US federal legacy systems GAO ranked most in need of modernization in 2025, eight use outdated languages and seven have known cybersecurity vulnerabilities.
Running legacy systems can require staff who know older languages such as COBOL, and GAO reports agencies struggle to find them.
About $83 billion, or 79 percent, of planned FY2025 IT spending at the 24 CFO Act agencies was for operations and maintenance; GAO says the legacy share of that is unknown.
GOV.UK research found that non-technical factors, not technical difficulty, usually stop legacy migration projects.
Legacy systems rarely split cleanly, so teams first look for seams: places where the system can be cut into parts that can be replaced one by one. For cloud applications, AWS lists seven strategies, the 7 Rs: retire, retain, rehost, relocate, repurchase, replatform, and refactor or re-architect.
For a website example of leaving a legacy platform, see migrating from Sitecore to a headless CMS. Budgets are covered in the cost of website migration.
Sources
- Updated definition of legacy ITGOV.UK (UK government guidance) · gov.uk
- GAO-25-107795: Agencies Need to Plan for Modernizing Critical Decades-Old Legacy Systems (July 2025)U.S. Government Accountability Office · files.gao.gov
- Understanding legacy technology in governmentGOV.UK Technology in government blog (GDS/CDDO) · technology.blog.gov.uk
- Strangler Fig (bliki, 22 August 2024)Martin Fowler (originator of the term) · martinfowler.com
- About the migration strategies - AWS Prescriptive GuidanceAWS · docs.aws.amazon.com