It's one of those things that's a little hard to say of course. We're still 40 years away from finding out. the obvious path for the chinese is to fill the area and surrouding areas with their population as much as possible and then when the time comes have them vote to 'seceed' and have china annex them, much like what happened with the Crimea. Which is of course exaclty what russia would do and why they're scared of it.
But - It will depend on the situation at the time. If it were right now - russia is badly weakened, economically devistated, and would be hard pressed to put up a real fight with the chinese. But will that be the case 40 years from now?
As it is russia's population is currently shrinking so we will have to see how things go, but i'd be more inclined to think the chinese would play an even longer game, take advantage of russia's economic weakness to gain further controls and inroads into their economy and be the iron hand in the velvet glove that helps keep them afloat while gaining more and more control, until it's too late for them.
Given a choice, China tends to buy their conquests rather than invade them.