finally...they called back and said, after reading through all the detailed rules again themselves etc, they found it's indeed allowed and so went ahead to do the re-routing for me - I just need to pay the change fee + extra airport taxes / fuel surcharge + the differences in fuel surcharge for HKG-TPE vs. original issue time.

so it really helps to know what the rules really are...I have experienced this in a few instances lately (e.g. a few weeks ago being refused the buggy coupon for a flight at gate 45+ when the Wing reception said KA flights were not eligible - which I mentioned in more details in another thread).