Answer: It depends on what you're hoping to achieve. It is possible to rejuvenate old trees with a program of pruning, soil enrichment, and pest management. On the other hand, if you are hoping for a predictable harvest, you may be better off starting over with new trees.
If you decide to keep your venerable old trees, plan to prune over a three year period. Do the pruning in late winter or very early spring. You can safely prune out up to a quarter of the crown of your tree; any more than that and you will simply encourage a flush of new growth.
You can prune your old grape vines right back to the ground; the roots will send up new shoots. Choose 3 of the most vigorous shoots to grow the first year; the following year, cut back to the one best shoot and keep that one properly pruned each year by checking the diagrams on grape pruning in books at a local library (it's hard to describe without a picture).
Q&A Library Searching Tips