Booked on a HKG-CX-LHR-BA-VCE, LHR-CX-HKG flight.

Wanna change my LHR/HKG flight into LHR-BA-PEK-CX-HKG, is it possible to do so? And how much would be the cost incurred?

I have already issued my ticket. And i have to re-issue it later due to the fact that i'm changing my HKG/LHR flight schedule, is it possible for me to change the routing during the re-issue in order to save the money?

Thanks