My new charge came through within a couple of minutes. Based on doing the same thing with CRT, I expect to get a refund in about a week. See you there!
The confirmation email says you can cancel, which is in direct conflict with the info on the website. Some people seem to have had luck with calling the Dining people, but I don’t have their U.K. number so I just did it online and it seems to have worked fine. It functioned exactly like a normal ADR: I picked the 7am slot and it warned me that I had a conflicting ADR (ie the 7:30am slot). I clicked the button to cancel it, and it went through exactly as if I were cancelling a normal ADR. I got a cancellation confirmation email and a confirmation email for my new time.
If you are concerned, I would do it the way I did, rather than cancelling the 7:30am and then trying to rebook at 7am. Because the system overwrites it, it doesn’t cancel your existing reservation until your new one is confirmed.
Edit: Is that clear? What I’m saying is: don’t cancel and rebook. Book the 7am as a new reservation and let it overwrite the 7:30am reservation with the new, 7am one.