I did this over the phone a few weeks back. So if you’re not local it might be worth calling and asking because it’s doable but a bit of a hassle. I had to purchase new tickets and make new reservations on the app while on the phone with the CM, then they put in for my original tickets to be refunded and I had to cancel the reservations tied to those tickets. Refund for first set of tickets came through about a week later.
I was a bit worried I’d screw up and cancel the wrong tickets or lose my dining reservations but it all worked out ok. So it’s not really an upgrade but got me that extra day I wanted to add on.