The advice not to dick around with the dealer by hiding your trade is good.
Buuuuutttt.... if you're a dickin' kind of person:
1. Go to Carmax and find out how much they'll give for your '15.
2. Go to the dealer who has the car you want, and don't hide the fact that you want to trade.
3. Let...
Sell outright?
Assuming you still have them, and assuming you have all weather tires, would you consider selling them? I don't need them right away, and I'd be willing to wait until you got a set of GT PP wheels. I'm in Southern MD, so it'd be a local pickup.