Getting to REP from PEK is not going to be possible on *A unless you go through ICN on OZ.
However, REP is a dirt-cheap flight most dates from BKK on Bangkok Airways or AirAsia, and is less than an hour in the air. If you can get them to refund the $200 change fees, and get you to BKK, you're almost certainly going to end up (financially) ahead buying BKK-REP as a separate ticket, though the pain-in-the-butt factor compared to your original itinerary is, of course, still disappointing...