Funny this kind a question pops up. I had the Missouri History Museum call me a couple of weeks ago saying that their 300 year archive gold cd's have failed and they want to shoot all their old photos onto film.
My experience is more in cinema, dealing with preservation of nitrate prints, etc. I can't tell you how many horror stories I've heard like this, that involve other types of storage mediums. BTW I would absolutely photograph each original twice. I understand this idea is a little unorthodox but if there's a way to do this right I'd love to know. I already have a general idea, my questions are now what is the best B&W stock to use, what kind of lens attachment would I need to get each photo to fill the frame (I know I'm working with a very different aspect ratio but I would think the resolution of 120 provides sufficient head room).