I should imagine it won't be too difficult so long as they have availability on the new dates you wish to go. Not sure whether they would charge an administration fee??
When i went I booked through First Choice who book direct with Disney.
To start with we were going for 3 nights in March, i then changed it to 4 nights and then about 4 weeks later i changed the date altogether to February. There was no charge at all as the cost of going in Feb was the same as it would have been in March.
I think if it is the same price or dearer then it is fine. But if your new dates work out cheaper then they will charge you for changing. (thats what i was told at First Choice)