I am the orginal owner of a 1967 J-45 that had an adjustable bridge. My bridge cracked fairly early and I had it replaced with a rosewood and bone. It was way too many years ago to tell you if there was a large difference but I never had issues after that. I remember my luthier at the time also saying that they started to use plywood bridge plates which didn't help either. If however, I was looking a an original J-45 and the adjustable bridge was still ok , I would not mess with it. I think you will get an improvement but I would not sacrifice orginality for that I think will be a small improvement. I also don't think that a bridge replacement will affect resale at all. Many have done this but I would still keep it orginal.
Steve