Unless you have booked a package with or without tickets included (which is handled by the Walt Disney Travel Company), I have always found the folks in Reservations to be quite accommodating when I have made changes.
One year - I think it was for my reservation in December 2008 - I first changed the hotel I was staying in (I went from GCH to PPH), I then changed the dates of my visit (made it one week earlier than my original existing reservation), added an extra night onto the beginning of my stay, and I applied an AP to it to get an AP rate, which lowered the price dramatically from what it originally would have been. I did all of this in 2 or 3 different phone calls! I am surprised they didn't shoot me! Not once was I charged a penalty fee (because I went through Room-Only/Central Reservations and not the WDTC) or told I couldn't do it. They were very pleasant. I was always told that you could reserve a room and if a PIN or a great AP rate came along, you could apply the discount and change the reservation. It's only when you have a package and deal with the WDTC (even if you don't know you are dealing with the WDTC) that fees start appearing and things gets a bit trickier. As long as you go through Room-Only/Central Reservations, you should be able to make those changes you need all the way up until 5 days before arrival.
Now, of course, this is all subject to change. DLR reservations may decide to change their policy at some point and not make it so easy to revise existing reservations. So what was once the case may not always be the case.
