http://www.bungie.net/7_The-Dark-Below-Preview/en/News/News?aid=12442
if you want your "old" exotics to do more than tickle crota, you have to spend an exotic shard to trade your gjallarhorn in for a new one with extra damage, and then level it up again.
http://www.gameinformer.com/b/featu...low-new-details-on-the-destiny-expansion.aspx
"Youll need three things. First, you need to own the original version of that exotic item, and turn it in to exchange for the upgraded version. It doesnt matter if this old version of the item is brand new or fully upgraded. To complete the upgrade, youll also need to turn in an exotic shard. This recently introduced new material is available for purchase for seven strange coins from Xur, or by breaking down another existing exotic item in your inventory. Finally, each exotic upgrade costs a hefty dose of cash around 7000 glimmer as of my playthrough of the expansion in mid-November. After turning all three of these things into Xur, hell hand you a brand new version of your item with a higher defense or attack value cap.
Its important to note that this process resets the upgrades on your exotic item, but youll now have a new higher damage or defense cap to work towards so youll need to decide if youd rather keep your fully upgraded old version of a weapon or armor piece, or start over. With that said, the process of upgrading exotic items is now faster, and does not require ascendant materials. Instead, the last slot requires one additional exotic shard, as revealed in Mondays patch."