- Audit what the legacy system actually does before assuming you know its full scope.
- Undocumented business logic buried in old systems is the most common migration surprise.
- Running old and new systems in parallel is safer than a single hard cutover.
- A phased migration by module or team reduces risk versus an all-at-once switch.
Audit before you assume you understand the system
Legacy systems that have run for years frequently do more than anyone currently at the company fully remembers or documented — a report a finance person quietly relies on, an integration nobody flagged. A genuine audit, not just a review of official documentation, surfaces these before they cause a mid-migration surprise.
Undocumented logic is where migrations get derailed
The most common source of legacy migration pain isn't the visible features — it's business logic buried in the old system that was never written down anywhere: a specific tax calculation, an approval rule with an undocumented exception. Finding these early, through interviews with long-tenured staff, prevents a costly mid-project discovery.
Parallel running beats a hard cutover
Switching everyone to the new system on a single day, with the old one immediately turned off, leaves no safety net if something's wrong. Running both systems in parallel for a defined period — even if it's operationally awkward — gives a fallback while confidence in the new system builds.
Phase by module or team, not all at once
Migrating one module or one team at a time, rather than switching the whole organization simultaneously, limits the blast radius of any single mistake and lets the team learn and adjust the approach before the highest-stakes pieces move over.
Plan the decommission, not just the migration
A surprising number of legacy migrations technically finish — the new system is live — but the old one never gets properly decommissioned, quietly running in the background as a cost and security liability. Planning the decommission step as part of the project, not an afterthought, closes the loop properly.