Definitely go ahead and try. I just changed some reservations for April and I was still able to get what I wanted (and we're going during a super-busy time). I would call asap and see what you can get!
I think it would also depend on where you want to get your ADR's for. It may be impossible for places like Cinderalla's table or LeCellier but I am sure you will be able to get reservations for alot of other places. Keep trying and be flexible in your dates and times.
Sure, if you're willing to eat at out-of-the-way places, or at unusual times. The real popular restaurants like Le Cellier, 'Ohana, and any character meals will be long gone, but there should still be availability at most resort restaurants.
i was able to alter my adr's last week...about 6 weeks out...w/o problem. AND it is Easter week! not only did i get what i wanted, but there were multiple time slots available. some of them are popular places too. so...good luck, you will prob get most of what you want!