Before, I thought "handset exchange" would be something where you bring in an old phone to get credit towards buying a new one. Like trading in a car. And like trading in a car, they'll want to make sure the old phone is actually worth something before they buy it from you.
Now... you're proposing a warranty exchange? Surely they check to make sure the device is actually eligible for the warranty (i.e. that you bought it, and within a certain timeframe) before they accept it?
Or is it something else entirely?
The fact that it would be worth the value of the parts is a good one, but not related to the question of whether Airplane Mode makes for a loophole.