Species-wide self-modification on that scale probably isn't something you should do unless you are certain that 1. it will work as intended, 2. you can reverse it if it doesn't, and 3. the change itself doesn't screw up your ability to determine whether it was a good idea.

I agree (at least with 1 and perhaps 2), but this answer seems like a cop-out. I think the point of that theme of Three Worlds Collide is about self-modification in principle, not just about it being safe and working as intended. The true rejection is not "We'd like to, we're just being cautious", it's "This modification repels us and we wouldn't do it even if we knew it would work as intended".