The question on replacing versus rebuilding the bottom bracket is one of labor cost. Personally, I find rebuilding an adjustable bottom bracket a relaxing way to spend an evening. That said, I just replaced the bottom bracket and cranks on my '88 Trek 520, with a modern external bearing combination. It does seem stiffer and smoother, but not enough to justify the cost, if that was the only reason I was replacing it. I don't consider this to be a C&V bike, so I don't worry about keeping it original.