Hey @Schuby, thanks and glad to hear it's resolved
The migrate_auth CLI command is actually intended to migrate all users to a given authentication service, so it wouldn't be applicable to convert one specific user back to e-mail authentication.
However, you can actually do this from the System Console. If you ever need to in the future, here's how:
1) Go to System Console > Teams, then click the "+" button next to "TEAMS"
2) Choose a team the user belongs to, then on the left-hand sidebar choose "Users"
3) Search for the user on the list, then choose "Switch to Email/Password" from the drop-down list
