I figure this will be the general consensus, but I presume that you cannot use Chopper to discard an upgrade if you are at full shields .
With the latest FAQ on Pulsed Ray Shields and R5-P9, you can't pay the cost (receiving an Ion token, spending a Focus token) unless you can also regain a shield. I'd presume Chopper works the same way. I guess Gonk could stand to be clarified in the same way, but the precedent on PRS and R5 seems pretty clear.
There woudn't be too much use to it with Chopper, but it prevents you from bringing BTL-A4 (for the rare times when you might actually want to joust with a non-R3-A2 turret Y-Wing), and then discarding it on the first round almost every game.