How many accounts share this email address?
Updating the email is pretty simple within most clients - you just specify the new address, and the client prepares/sends the payload. You can do it all from one machine if you can collect the AccountKeys from each installation.
If you're looking at 20+ accounts, this could be scripted pretty easily. The following thread has at least 2 scripts (1 go, 1 ansible playbook) that are designed to check PrivateKeys are AccountKeys. IIRC, only the endpoint and payload would need to be modified to accomplish your goal: