This happens on a regular basis (good for me). I just either do:
1. change flight to the same reduced value and get the credit, then change flight back to original with $0.00 due
2. book the lower one then cancel the other
This happens with UA and AS for me since most of mine are booked (LAS-LAX & reverse)- in fact this morning had this on an AS flight in October. I did #1 above.