Cleaning up users table after new email domain

Company I work for just changed names, and email address domains. This
has led unfortunately to our RT system now having autocreated users with
these new email address accounts, as well as the original accounts.

Question: Is there a tool or procedure that can help me “collapse” these
new accounts to the original account names? In other words, if “userx"
exists, reset the tickets etc. for "” to “userx”?

Thanks for any references.

Alan Sparks, UNIX/Linux Systems Administrator