One thing to note is that the imperialist powers didn't start out either benevolent or workers' states. They were always or always tried to be colonial, capitalist powers. It's unlikely that Chinese communists would manage to dupe over a billion people into supporting it's rise, only to spring imperialism on them later.

Plus, building worker power to the extent that China has, teaching Marxism to the extent that it does, will naturally create a lot of strength in the people to fight the hypothetical change. The chance to change course without massive domestic repercussions is slim. The western trope is that Asian people are subservient, hence easily manipulated by a 'totalitarian', 'authoritarian' party. But that's not the case. It only works because the people there are 1. aware and 2. supportive. I would expect the same kind of resistance to becoming imperialist in China as there is to becoming socialist in the west.

Outside China, multipolarity undermines imperialism, if doesn't necessarily give the power of the US to China. The US maintains it's hegemony through military and international institutions such as the IMF, World Bank, and WTO. We would notice China becoming imperialist long before it was a remote possibility. For a start, the western world would have to become financially and militarily dominated by China.