Tenant to Tenant Migration - New users and existing guest conflict

I'm trying to find the best process to migrate users from tenant A to tenant B.

Users in tenant A already exist for the most part as Guest Mail Users in tenant B.

Provisioning/prestaging of tenant's A users in tenant B is done through the creation of accounts with a DNS suffix in tenantb.onmicrosoft.com (eg: usersA@tenantB.onmicrosoft.com)... but when i'll switch the domain name used by the users of tenant A in tenant B (cutover), I will have a conflict to reallocate the domain name on the UPN of provisoned users because a guest account with this email address already exists...

Does anyone know the best practice for this kind of situation?

Thanks !