MSPeconomist

If son is only 6, presumably a parent or nanny will travel to bring him back from HKG at the next vacation or end of the term, so if you know the date, you might want to book that ticket at the same time as you make the change. That way there will be no issue about the reservation looking like the boy will be traveling alone, possibly at an age too young for travel as an unaccompanied minor on this route. You need to split the PNR and then make changes, liking the son's changed ticket to the new adult ticket.

Depending on your fare rules, you could lose the remaining value in the child's ticket if you don't make the change (or at least split the PNR and cancel the reservation) before the flight. If he just no shows, you should still be permitted to fly on your ticket, but you might not be able to use your son's return ticket later. However, this may be moot if your son's later travel will violate the maximum stay rule in his fare, so you should look for this in the fare rules before planning to use the remainder of the ticket much later. It might even be cheaper for the boy to return to the UK with you now and travel again to HKG at the end of January, depending on the dates involved and the minimum and maximum stay rules that CX uses on various fares.