Note that, if you try to make the change online, the system may allow only a free change -- if so, you'll see a banner telling you of the specific criteria. In order to recover any points difference (if the new flight is cheaper), you'll need to cancel the old flight and book the new one from scratch (not use the change flight function). I'd recommend booking the new one first, to make sure it's available at the fare you expect, then immediately cancelling the old one.
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