[eevilspock]

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.