The principle of delta pass is that already migrated items are never migrated again to avoid duplicates.
Therefore, you cannot reuse the same license to migrate 2 times the same mailbox.
If you want to remigrate a second time the same mailbox, you have to delete the user from the cloudiway user list, recreate him and assign a new license.
From the platform point of view, this will be considered a new user to migrate and it will be entirely remigrated.
First, you will have to empty the target mailbox because items will be migrated a second time.
Same logic applies accross all the platform and is the same for Drives, Sites, Groups and Teams migration