Not quite sure they are 'full' -- we have seen ONE post about one resort on certain dates being 'somewhat full' -- but I doubt very much that's the case at all resorts at this point.
And in years past../ some resorts (like Pop Century) and certain others that are very popular had very little F/D room inventory ... because they were so well 'pre-booked' with F/D hopers. (hope-ers) I suspect more than a few folks who booked now to secure "their favorite resort/view" will be forced to move in the fall for whatever promo comes out.
Modifying your current reservation WILL ONLY work if the resort you are currently booked at has rooms allotted to whatever the fall promo is.
If you don't - you'll be rebooking at a new resort. Technically it's not a cancellation per se -- but what you have booked now guarantees you nothing in relation to the promo except possibly your ADRs.
If you are able to switch your reservation around later - you would have just as much luck making a new reservation at that same time.
Regardless.. people feel the trip is more 'real' when it's booked now -- but I cannot stress enough now - booking now guarantees you NOTHING for the fall promo and actually reduces (however slightly) the promo room inventory at that resort you MUST have.
I keep repeating that. Not because I'm a jerk.
But because every year when the fall promo is announced - people get all bent out of shape because they pre-booked in anticipation of the promo -- thinking they were ahead of the herd -- and then they have to switch resorts and possibly dates because they weren't aware that the promo inventory is completely separate from what they already have booked.
Book if you must - but realize the prebooking for what it is -- something that makes you feel better - not something that actually guarantees you anything.
Knox