Currently when editing an existing event registration (participant) record - we FREEZE the Event Fee(s) element if there is an associated payment (contribution record). This is correct behavior. HOWEVER, we allow the user to change the EVENT which the person is registered for. This is not good as it basically invalidates the contribution record / amount paid (and in fact results in displaying the new event fees in a frozen field with NO event fee selected).
We should also freeze the Event select field in this condition.