In my understanding, in an ordinary situation, changing an X (transfer) to an O (stopover) is considered re-routing and needs a reissue (as one needs to change the "X" to an "O" on the TKT + change the Fare Calculation and add the tax). However, I know ex-TPE CX is able to bend this rule for stopover in HKG (but not anywhere else) and simply collect HK tax upon check-in. Whether AM can do that I do not know.