How to match objects in a merging / synchronization project

Hi there,

currently I'm in a new project where in the target domains already the source accounts have been created.
Some former aborted QMMAD migrations have been started, for some domains simply new secondary accounts have been created for the users to provide different resources.
Goal would be to merge and synchronize the existing accounts and afterwards to remove the source accounts.

But it looks like that we cannot use SamAccountName, SIDHistory, nor Email for object matching, because all are used and different.

Is there any other possibility for the matching, or do I have to make it work with one of the mentioned attributes?

Kind regards,