Bring an app you already have
Step-by-step guides for moving an existing app here — its code, database, files, secrets and domain — organised by what the app is built on. Your code is standard, so there is no lock-in on either side: you can bring an app in, and you can take it out again.
What a migration involves
Every app is (1) code, (2) a database, (3) secrets and environment variables, and (4) a domain. Move them in that order. Your code is standard — React, Python, Flutter and so on — so nothing is truly locked anywhere; the work is mostly copying and re-pointing.
Never switch the old app off first. Rebuild here, connect a copy of the database, test until it matches, then re-point the domain. The original keeps serving your users the whole time, so a failed attempt costs you nothing but an afternoon.
Keeping your existing database or file storage and simply pointing the new app at it is a perfectly good first step — add its connection string under Deploy & Manage → Secrets and you are running here with zero data migration. Move the data later, at your own pace, once you trust the new app.