I don't see an issue with any level of spare parts, including a new chassis. (I certainly wouldn't call Cosmic a different robot with the new chassis). In fact, many robots don't even have a single part that could be called the chassis (Valkyrie for instance), so you couldn't restrict chassis replacement even if you wanted to.
Actually, after much thought about this issue in the past, my view is that it's the design rather than any physical parts that make a robot what it is. I would have no problem with someone having two identical robots, and simply running the second one if the first was damaged. Assuming you allow spares, what's the difference if they are assembled or not? And if a full spare robot is banned, how assembled can the parts be? It's a huge can of worms, and there isn't an easy answer.
I definitely agree with Shakey that these contentious issues are for a future rule debate. The current ruleset is a long way out of date, and I'd rather get something new in place soon, rather than waiting for it to be "perfect".