When you run cloud migration, at the end you can map on prem users to SaaS users and it renames the cloud user to match the old on prem one, meaning the relationship to some tables eg User Setup works.

However, it doesn't seem to update the GUID of the SaaS users, so the "created by" system fields all through the database don't relate to a user anymore. This is a problem for some customers as that audit trail is gone. When the user is renamed by the mapping process, it should also change the GUID of the new SaaS user to match that of the old on prem one.