Hi @Lorenzo Rovere​ 

You can use the DB migration tool - https://github.com/mrchristine/db-migration

For eg., copy the notebooks of user1@olddomain.com to the folder of user1@newdomain.com, then the latter can execute the notebooks.

If you are unable to log in to the workspace of the old domain even for an admin user, then please raise a support ticket to address this.