Convert user to mailuser for cross tenant migration

We are doing a cross-tenant migration from users in a source cloud tenant "source" (m365) to a new cloud-based tenant "target" (m365).

We follow the cloud tenant migration guide from MS from here.

In the source tenant, all users exist as regular mailbox users.

In the target tenant, all users already exist as regular users and are already using their accounts e.g. for office applications, Azure, Teams, MFA. They were created as regular users and had a mailbox automatically attached (Exchange Online (Plan 1)). This mailbox is not used. So we do not want to delete/recreate the user accounts.

According to the documentation the users must be MailUsers for the migration to work.

- If i remove the exchange online license from a user, the mailbox is removed from the user and he becomes a regular User in Entra. However i cant figure out how to convert this user to a mail user (the Enable-MailUser cmdlet only works in on-prem instances).

- I don't know if i can directly convert a mailbox user to a mailuser (without removing the Exchange license).

I am currently stuck and out of ideas and are very thankful for any suggestions from your side!

Thank you very much!

Georg