As long as you are at least 60 days in advance, you can cancel or change your date without penalty. If you book for a different date, the rate that applies is the rate that is in effect when you change the date. That may be a higher fare than when you originally booked.
I think I was told (by Donna) that if you change your date, you still get the 10% off and the stateroom credit. It is just your base rate that may change.