Self-rescheduling for players in the member portal

Currently if a user registers and pays for an event and subsequently changes their mind so that they want to register for a different event, the only option is to withdraw (get a refund) and then register for the different event (pay fees, plus processing fees that we eat). Instead of dealing with refunds and subsequent registrations, a user should be able to transfer from one event to another, applying the credit from the withdrawal towards the new event. Depending on the scenario, the user is charged if the new event costs more than the original event or is credited if the new event costs less than the original event.