No downside to your plan as far as I can tell. I'm not sure about AS fare construction, but often fares (other airlines) will reprice using historical fares if done when the trip has already started. Shouldn't matter really in this case as first of all, you will not make a change, but cancel, and secondly AS 'return' fares are usually constructed from multiple one-way fares.

BTW: why don't you just try to change your trip now on the website, and it will tell you what the refund will be (before you commit to it).