Just call them.

It's certainly possible "in general" if the cities in question are lumped together in one zone, like TPE/RMQ are. I'm not sure if that's the case for PEK and CTU.

But those ex-China I fares are known as the cheapest business class fares in the network, so I'm not sure. Maybe there are some restrictions on the ticket conditions preventing a change. Let us know what they say.