Your trade won't de value if you have signed the deal then wait for it to arrive. I signed my deal with a trade and the car came 3 months later and with 7000km more on the car. But the deal was signed so they had to honor it. If the dealer is worried about that then they might give you a loaner car and take your car. I think someone here did that.
Of course you always run the risk of wrecking your car while waiting for the new car to arrive... Then the deal won't go over so well anymore.