One thing to bear in mind is that when you change a date on a reward flight, it often requires the booking to be reticketed. You may a) lose seat reservations already made or b) be unable to reserve seats online until reticketing has completed (which can take a while). This is obviously not a problem if you're not bothered where you sit.

In my case, the cost of the change was almost completely offset by changes to currency/fuel fees (I can't remember which).