Hi @orthoguy! Unfortunately, there is no way to change the email someone registered with at this time. The best thing to do would be to recommend they register with the email they would like to use.
If you want a cleaner registration report later down the road you can remove the old email they registered with similar to the instructions included in the highlighted section of Managing meeting and webinar registration.
Thanks for the reply. This is a paid webinar, so I'm guessing I'd have to cancel him, refund his money, and ask that he registers and pays again, meaning I get charges 2x credit card fees, or is there a more elegant (less costly way).