Because it's easier on the back-end to do it like this... and it keeps you playing to grind up the levels. Basically you currently have say the Hard Light (refID#11211), there's now a second "new" item in the game (refID#21141) with the same exact name that you're exchanging it for. For people with maxed weapons, they could've just handed a max weapon back, but most people wouldn't think enough to go into the skill tree to reset it the way they had it, so the feedback forum would blow up with "mah gun doesn't act the same", or take up some database scripts to transfer what you've selected before-hand to the new perk ID of the "new" weapon. For the players that don't have maxed items, again there's the whole thing of either setting incorrect perks or running a database script to figure out what perk you had, which is further complicated by the fact that the new exotics take less XP, so the perk XP will be different, meaning you could have a perk unlocked on one, but not the other (even though they could've just left it as an open perk column like you haven't chosen yet). As to your question of why not just add a node to the tree (as I think everyone assumed they would do)... think it comes down to less to do tonight when the DLC goes live, they can already have the new weapons in place and ready to go in part of that 2GB update last week, and just update loot tables to the new refID# when the servers reboot for the update tonight. ...I probably lost 99% of you in that wall of text :P