During our August trip to WL, the front desk left a message that we could book extra days at the special "Stampede" rate.
I didn't even ask what it was, as we had to sail from Port Canaveral on checkout day.
And if I didn't have so much $ wrapped up in the cruise, I might've taken it, we loved WL so much!
But, as has been suggested, you really probably ought to just go ahead and book it ahead. The anxiety of changes is a tad too much for me sometimes!
Good luck!