What is the best way to shorten a driver then reweight it
The simplest and best way is to remove the grip, cut to the new desired length, then install a new grip. Since you’re probably trimming a graphite shaft, be sure to tape the shaft where you want to cut it, and use a fine tooth hack saw blade so you can make a clean cut.
I assume when you ask about reweighing it, you are referring to the swingweight. Expect the swingweight to be much lighter – about 3 points for every 1/2 inch you shorten it.
Here’s a link to articles for how to do club repairs – http://www.golfworks.com/clubmaking-help-and-info/a/1652/
Hope this helps you! Michael