This is a user-to-user forum.
Apple Trade In - Apple
That said, Apple's FAQ seems to indicate that with respect to the iPhone 16 Pro Max, "If you haven’t dropped off or shipped your device yet, you can cancel your trade-in by simply keeping your device."
With respect to the iPhone 14, I don't know if you could substitute it in time to apply any credit towards the price of the new phone. If you bought the new phone first and then traded in the iPhone 14, that might leave you with a gift card whose value could only be applied towards the purchase of more stuff from Apple.
You're going to need to contact Apple if you want an answer to your question.