My solution would be to have Xur or the gunsmith sell some sort of item based on its item type (example:hand canon barrel, auto rifle heat sink, helmet reinforcements) they could alternate the availability of these items with a vendor. But the main idea is that it will bring one legendary item up to the level cap. They could make you level it up again or not, either way I would be satisfied. Perhaps they could even make pricier upgrade items that would upgrade an item without you having to level up your item again