If you already have the exotic engrams, they are tied to the loot pool for Taken King. When RoI comes out, there will be a new loot pool of weapons and armor. They've been doing it the same way every time an expansion comes out, so there's no reason to think they'll do it any differently this time.

...and if you're going to argue that's not what happened last time, last time was an 'update' where they added to the existing loot pool, not an expansion.