You're right Helen, they can be used for collaboration, and that was probably their original intention, but I agree with Jack here... collaboration with these comment boxes are awkward at best, and almost useless otherwise -- especially when replies are split across personal profiles.
There are no rules -- including no rule that you are supposed to reply on the other person's profile (sorry Jack) -- nor any enforcement, and everyone uses them differently. Comments tend to never get deleted, as there's no revision history, and they end up filling the page, looking ugly.
I agree with Jack -- just make the change.
Otherwise, collaboration is often best done either here on G2G (also a bit awkward though), or on a designated Free Space page.