Settings disappear after downgrading from Pro to OSS

My Nexus version is 3.40.1
I’m following the “How to downgrade” document, and after downgrading – all my settings, users and repos disappear and I’m presented with an empty OSS instance and the Admin password dialogue. When I revert back to Pro, the settings, users and repos are back.
I have a S3 Blob storage in use, should be OSS-compatible.
Realm-wise, I don’t use any Pro authentication plugins.
How do I migrate the settings?
Thanks in advance!

UPD (edit to add): I believe I found the problem – there is nexus.datastore.enabled=True in the config, and an external Postgres DB configured as a datastore, which is Pro only.
Is there a way to migrate back to OrientDB?