1) The reason why such a refactor might be necessary is from things the first group tried but didn't quite work as intended, or that the users used the system differently to intended. The first group has that knowledge, the second group doesn't. So the first group will do the refactor better than the second group.

2) Beware of Second System Syndrome https://en.wikipedia.org/wiki/Second-system_effect where everyone tries to put in every feature that was missing from the first system, simply because there is no urgency around the second system, because the first system is already running.