Yes, absolutely. As long as you are within your time limits to cancel - I think the cutoff is 30 days pre-trip for packages and 5 days for room only (?) - you can do so with no penalty. Here’s hoping we get some good discounts in Jan-Feb because rack rates are insane. Watch the Discount Codes and Rates sub forum here for news.
Resort discounts can get snapped up very quickly the morning they are released. Inventory changes by the second. If you see something you want, grab it as quickly as you can. On discount release morning I will typically just book a new reservation because it’s often quicker than going back and trying to modify what you have, not to mention far far quicker than calling. This is particularly true if you are changing more than one aspect of your reservation, say both resort and discount offer. The room you wanted may be gone by the time you get through all the modification process screens. (This has happened to me

).
Now, once I have a new discounted reservation in hand, I still watch inventory and modify it if something better pops up for the same offer code. Modify works pretty well for that.
Always rebook first
then cancel the old reservation. It’s OK to hold more than one resort reservation at a time. When you cancel the one you don’t want it usually only takes a few days for the refund to hit your credit card.