Hi Jay
There is a way to setup it. But it requires registrants to have an account on your website to update his registration record.
1. You need to create a menu item to link to Registration History view of Events Booking
2. In case users use offline payment method (he has not paid for the registration yet), he can click on that menu item, see his registration history and update the registration record with the option he wants. However, if the status is Paid (he made payment), he could not changed it because we made payment already, so there is no reason we allow him to change the options (the fee options) he selected and paid for
Tuan