So check price differences between an ATL stop and a RDU stop and make the change before departure if they are the same/less?

If making the change before, would it be better to do at the CX counter at MEL or at NRT? I'll be in NRT in late September, about 3 weeks before departing the DONE3.
Otherwise make it in HKG? (First flights on DONE3 are HND-xHKG-JFK)

The CX Tokyo office issued the ticket.