A Treasure Key is not considered the same thing, so there is no exchange. The only thing Treasure Keys do now is give you Reputation when you dismantle them. You'll have to get Skeleton Keys from the Nightfall (guaranteed on first completion for the week) or randomly from Heroic Strikes. Sorry. FYI: you still get loot, Skeleton Keys just give you an extra loot drop, which is one of the Strike-specific items.