Is it possible add a return flight (HKG-LHR) separately booked if at the time of booking outbound using a 241 the return date was within booking range (so not outside 355 days) - will this just be treated as a change so full tax on HKG-LHR payable (plus £35 change fee) vs a refund of 1/2 Avios for HKG-LHR (as would be the case on a >355 day booking)?

Thanks very much