Ex-TPE, one way is 0.6*round-trip for 1Y (0-365) TKTs. I believe you have a D1YTW (or D1YIB). You can calculate the difference that way. Note that I am not sure whether you are allowed to refund the return portion. I've never tried myself. Usually a re-routing means that you pay more if the fare is higher, but you don't get any refund should the new fare be less than the old fare. I don't know if this applies in this case.