It's completely optional and is only introduced due to the rarity of exotic drops in the game.
Bungie is giving you the OPTION to upgrade your current Exotics and trade them in, rather than roll the dice and earn a new weapon during gameplay.
OH FOR SHAME.
The Legendary drops also go up the 331 Damage and Legendary gear also goes up to 33 Light, so you don't even need the Exotic stuff unless you're trying to hit Level 32, which won't even be available until January.
So much complaining over a concept you guys don't even understand. The new Exotics START at 302 Damage, meaning even if you had a MAXED out Exotic, the new one is better, and won't use ANY Ascendant Energy to level up.
Just did SUROS and IceBreaker last night. I'm loving the new content, especially the PvP maps.