Looking for info on pruning 20 yar old overgrown concord grapes. Can I really cut the many very thick and very long vines down to just a few and start the trellising process anew, or will that kill the plant entirely? The main trunks are from 2 to 6 inches across, and some of the "cordones" are twenty feet long. The vines produce some grapes and lots of foliage. I would like to manage them/renew them and produce a larger yield.