Yes, I have to sign it with my release key to make another gplay_full APK.
I’ve recently added the flag “ask the user when uninstalling whether to preserve the data or not” (v1.16.0.6). That would ease migration to the github/fdroid release channel a bit. Android by default kills (Syncthing synced) user data on uninstall if it is in the /Android/data/ folders. So I’d suggest to wait until v1.17.x is ready (do not use my preview version v1.17.x as this is for my own tests). Then you could fully sync every Syncthing folder as a precaution, do backups of your data (just for the oh no case if anything goes wrong unexpectedly) and use export/import to switch to the fdroid release.