Some excursions flat out say Sold Out. If you have fast fingers and are daring - and it’s not at the moment a booking level opens, you could cancel what you have, try to book the other; if there aren’t enough spaces, quickly go back to pick up what you had.
There is no computerized waiting list. If you cancel and it gets reserved by someone else in the short time it took you to make a few keystrokes, it is a coincidence of timing.
Definitely not something to do when the next set of cruisers are getting their first chance at excursions.
When I look at my reservations, it will let me see the times that the excursion is offered (if all slots at all times are not taken - then the excursion moves to the Sold Out section at the bottom of the page.
I did this the other day for a Sep cruise, somewhere between Platinum and Gold booking. I cancelled, selected our names, and saw 2 of the 3 time slots as available. I knew ahead of time what I would accept, so if those slots were not available, I wouldn’t waste time making that decision - so I could go back and pick up my previous reservation. You can miss out if doing this.
(Edit - just reread
@cvjw ‘s post which used so many fewer words).