Got to understand that on a retro-fit, you don't know what you've got till you're prepped to cut it out. If you don't have the materials to cover each scenario, you're gonna be at a standstill or (improvise). With what you're dealing with today there's no room for improvising and ending up with a quality result. There's a lot of mix-n-match crap out there and you get to deal with it as it comes about. As-builts don't always tell it like it is. It's being upgraded for a reason!
Later, Mike M.