Recommended process for credit card additions/'edits' to event registrations?
You're not the only one looking for this solution, but it has been unfunded so far because it would be so complex.

As an alternative, you might consider separate events, either using the Event Additional Signup extension or the new conference session features.  People who come back later and want to sign up for an additional option can just sign up for the event directly.

You'd then have a different, but much simpler, customization to build out: just getting the appropriate events onto someone's badge.