Perhaps the best way to 'upgrade' these weapons would be with a new drop from a Super Hard version of the same raid? So you're still relying somewhat on RNGesus (which would keep Bungie happy), and you still have to earn it by playing through the Raids again. Then, take them to the Gunsmith (as suggested above) with a crap load of glimmer/upgrade materials and let him do his thing. Perhaps the Gunsmith could even take the gun away from you for a few days/week so you have to be without while he carries out the upgrades? I also love the idea of raid equipment being useful again. So we don't have to all look virtually identical if we want to be at or near the max level cap.