Honestly, this is a good idea. All they need to do is treat them like exotic weapons where you trade in your old plus a new raid weapon and glimmer in order to upgrade to a new version and start from scratch to re-level it. The problem with just allowing you to upgrade the old weapons is because of progression gates. They're annoying but they're a vital process for the MMO genre (which you can make an argument either way on for Destiny). If players immediately hit the new level cap right after the DLC hit it would cause a lot of people to quit prematurely. So upgrading your old legendaries should neither be easy or cheap. The exotic upgrade process was done well I think.