After some investigation, it has come to my attention that there is a known issue with the Mover IO migration tool, which is currently impeding its ability to identify users in the new tenant. Microsoft and Mover IO are collaborating to rectify this issue, but as of now, there is no definite timeline for a resolution. However, while awaiting the tool's fix, there are a couple of alternative approaches you can consider to proceed with your migration.
Firstly, you could explore using a different migration tool. Several alternative migration tools, such as BitTitan MigrationWiz and Skykick Migration Suite, are available. Switching to one of these options might help you migrate your users without the roadblock you're currently encountering. Another viable but more labor-intensive method involves manually migrating your users. Although it demands more time and effort, manual migration is a feasible option. Detailed instructions for this approach can be found in Microsoft's documentation.
Ensure your new tenant is meticulously configured before initiating the migration process, including domains, users, and permissions. Pilot the migration process in a staging environment as a preparatory measure. This allows for the identification and resolution of potential issues before undertaking the migration in a production environment.
Lastly, always have a contingency plan in place to address any unexpected hiccups during the migration process. This could involve maintaining a backup of your old tenant or implementing a procedure to restore data from backup in the event of a mishap.