Would it be legal for congress to allow a territory of the united states to become a full state on condition that its residents choose a new governor?

No, it would not be legal.

Explanation:

It would be a violation of US law if Congress allowed a United States territory to become a complete state on the condition that its residents choose a new governor. This is because the US Federal Congress is prohibited from limiting the independence, sovereignty and management of a state over its territories. Thus, allowing a territory to become a complete state is a way of limiting state political power within that territory, which means that it is illegal.