Does all the old butyl tape need to come off?

AnEv942 has it on the money. Take the whole thing off when you can, and clean the butyl off with a scraper then MEK (methyl ethyl ketone). Basically the only way to be sure is to get it down to the barest possible and remove all glues and caulking. Once it's all back together with new butyl, a little eternabond on the seams isn't a bad bit of insurance, but it needs to stick to fiberglass, wood, or something other than caulks and glues.