Mass Update 'Is Done' for Completed Project Tasks?

Thanks to a well-meaning but not-well-understood configuration setting during our initial setup, I find myself with 15k+ tasks where the 'Is Done' setting is False, but the Project Status is 'Complete'. This makes the My Work page utterly unusable for the majority of our PMs.

This leaves thousands of tasks per PM flagged as ' This has been assigned to someone else, and you don't need to work on this anymore." with the only resolution I have at the moment being to click the 'Remove' button 15k+ times to clear their profiles. While I can write reports that filter Work Items to find my problem tasks, I have thus far failed to find any solution for a mass updates to solve my challenge. Asking my PMs to click 'I'm Done' 15k+ times is a non-viable solution.

I understand I can configure the Team setting so that clicking 'I'm Done' marks a task complete, but the reverse seems impossible. I can easily train my PMs going forward, but I don't think it's reasonable to expect them to cleanup the existing mess since our launch configuration created the issue in the first place.

Any clever tricks anyone else has up their sleeves? Fusion solutions are a viable option if possible. Solutions that involve reassignment of tasks are viable as well.

Katherine