"Get it right with the right functionality", that would be my motto in this case. Functionality would always supercede interface (in a forum such as this), but it wouldn't supercede useability. Like you said, it's an irreversible "promotion" so to speak. If the current forum has more functionality than the new, then I would hang on to the old. It does its job, the majority of users haven't complained about the layout but are talking more about functionality. The new forum seems to focus more on looks.
The current forum is good enough, in my opinion.