I changed a couple of mine last week, and had no problems. I booked the original ones a couple of weeks ago. I got every place I asked for and all within 10 minutes of the time I'd like to go.
I agree with booking soon, if you decide to change something, you can call, see if it's available, book it, then cancel the original. But don't cancel until you get the one you are looking to change.
Party size does seem to make a difference. We are 4, but some ADR's are just for 2 people, those seem to be easier and quicker to book.