Short answer
Legacy system migration is moving a business off software it has outgrown, such as an unsupported CMS version or a custom application nobody remembers building, onto a current platform with its data intact. The hard part is rarely the new build. It is the data, the undocumented rules and the cutover.
Also called: System migration, Replatforming, Modernisation
Old systems encode decisions that were never written down anywhere else: which discount applies to which customer type, what an inactive account means, why one field has been repurposed to store something unrelated. Those rules surface during migration, usually as data that refuses to fit the new schema. Budgeting for discovery and data cleaning as their own phase is the difference between a migration that slips a week and one that slips a quarter.
Cutover strategy is the second decision. A single switchover is simple and unforgiving; running both systems in parallel, or replacing one function at a time behind the old interface, costs more and lets each piece be verified before the old one is retired. For anything public-facing, map the old URLs to the new ones and redirect them, because a migration that changes every address without redirects discards the search visibility the old site had accumulated.
Common questions
Will migrating hurt our search rankings?
It can, and the usual cause is unmapped URLs. If every old address redirects with a 301 to its new equivalent, page titles and content survive the move, and the sitemap is resubmitted, most sites see a brief dip and recover. Launching a new structure with no redirect map is where the lasting damage happens.
Where this comes up in our work
Related terms
Technical Debt
Technical debt is the future cost of a shortcut taken now: code that works but is structured for speed rather than for change.
Database
A database is the structured store behind an application: customers, orders, bookings and stock held in a form software can query reliably.
Content Management System (CMS)
A content management system is software that lets non-technical staff create, edit and publish website content without touching code.
XML Sitemap
An XML sitemap is a machine-readable list of the URLs a site wants crawled, usually with a last-modified date for each.
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