I don't think you necessarily need an "AR" gunsmith, a 'specialist'... any competent gunsmith should be able to thread a barrel. Removal and installation of barrels is not at all difficult; its easily accomplished by the home assembler with the correct tools.
Bottom line, search for gunsmiths in your area, and talk to them. Anyone who offers threading services should be able to accomplish this.
Unless you have a "premium" quality barrel that shoots very small groups, you may be money ahead selling your post ban barrel to someone in a state that still requires them and buying a new threaded barrel.