This document showcases how you can force an email change
When using the How to bulk change users email address there are situations where the target email has already been used. For example, a re-org happened, and users created their email addresses. This will result in the change not being possible. However, you can force this change by ensuring that you’ve selected the force indicator during your scheduled task.
By doing the above, this lets the app create a dummy email address, force the change and send the dummy email address for deletion.