Downgrade your collection, which is necessary if you want to open it with an earlier Anki release. If you skip this step, you may get an error message when opening your collection in an older Anki version, and you will need to return to this version, downgrade, then try again.
Is it possible to automatically downgrade currently viewed profile on exit?
You might want to consider using AnkiWeb to sync your data between devices, as it is easier. and does not have the risks associated with manually copying files about. If you must do that, the recommended way is via an Export+Import, which takes care of downgrading for you.
If SyncThing is running while Anki is open, you risk file corruption by doing that, so I do not recommend it. But if you are determined, you can just upgrade to the latest AnkiDroid alpha and enable the new backend, and you’ll no longer need to downgrade.