Moving a patient-facing platform is like replacing a plane's wings mid-flight. You can't tell patients "sorry, the system's down while we refactor." Every minute of downtime has real consequences for real people.
We used feature toggles to migrate slice by slice. Weekly cut-overs felt mundane — which was exactly the point. No drama, no heroics, just steady progress.
A year later we bragged more about how little drama we had than about shiny tech. In healthcare, boring is the highest compliment you can give an engineering team.