No. At the very least the folder needs to be removed on the receiving side, and unshared on the sending side. Restart the sending side before setting up encryption towards the receiving side. Hopefully this should clear out enough data to make it work, but probably noone tried this maneuver yet.
I’ve said this before and I’m not precisely impartial, but I copy data using Syncthing to a couple of differently located filers that use zfs and periodic snapshots. This has been my backup strategy for literally decades (though using rsync in the pre-Syncthing days) and it’s been adequate so far. Syncthing is not a backup tool, but can be a component in a backup solution. When used with care. I’m currently experimenting with making some of this use encrypted folders. There are pros and cons to that obviously, the cons being in the monitoring and restoration parts of the story…
True, but that’s also a common reason people don’t do backups at all. What I’m noting is that you could look at it not as a disadvantage but as enforcing best practices.
The thing is, it seems to me like even tools like restic are components, rather than the whole story. The odd thing about Syncthing is how many people seem to think it’s the only piece they need. Maybe it’s because the commercial sync services tend to encourage people thinking they’re good for backups?