I think some clients (like the server) must be able to sync with very old schemas because the user might have forgotten their phone in a drawer for two years. Each client just migrates itself asap.
I don't think any user who leaves their unsynced data sitting in a drawer for more than a year can reasonably expect it to sync to a shared database, not just because of schema incompatibility but more so because of conflict resolution (imagine expecting your PR based on a two year old git commit to be accepted to a very active project). Just give them a report of the diff (the unsynced changes) so they can redo them via the latest client manually.
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I think some clients (like the server) must be able to sync with very old schemas because the user might have forgotten their phone in a drawer for two years. Each client just migrates itself asap.
I don't think any user who leaves their unsynced data sitting in a drawer for more than a year can reasonably expect it to sync to a shared database, not just because of schema incompatibility but more so because of conflict resolution (imagine expecting your PR based on a two year old git commit to be accepted to a very active project). Just give them a report of the diff (the unsynced changes) so they can redo them via the latest client manually.