The fare difference will most likely be between USD 1000 and 2000 if it can be calculated at all (depends on your fare basis). Having flown F HKG-TPE dozens of times, it really isn't worth it esp. as the meal service has been downgraded to be similar to J. Also some chance of EQP so that no F operates on your flight. Good luck, if you are determined to do this then you will either have to go landside to the service desk or sometimes the F lounge manager will do this for you (but if you are only QFF gold than no F lounge access).