I've done it (and will be doing it again for our October trip which is already paid for ).
I've never done it less than 45 days out (so not sure if the rules are different), but it's super easy to do. I just called reservations and told them I wanted to switch and it was no problem. No penalty/fees, either.
It's easy to modify your reservation, provided the resort/room type you want to change to is available. For packages, any changes made 44 days or less from arrival will incur a $50 change fee if the total cost decreases (this fee is usually waived if the total cost increases).