They won't find out right away, and after all the paper work is signed they can't do a thing about it. Like charging you for the new cats or deduct it from your trade in value, they can't do that. All my trade-ins they never inspected the under carriage of the car,they do that later on before they put the car up for sale or auction it off.
Just trade it in and don't say anything about it not having cats, you will be just fine.
By the way, what new car are you getting?