I think part of the reason that's relevant is just trying to understand.... If the old pay system had been allowed to continue past the implementation of the new one, so it was available as a backup, that strikes me as the obvious best practice. It seems to me the reason that wasn't the case is that the consolidation was happening and the two projects weren't in each other's critical paths.
Consideration wasn't given to the fact that pursuing the consolidation too quickly would remove the possibility of preserving the old system until we knew the new one was working. Is that a fair characterization of what happened?