This is odd - why then wouldn't Tesla unlock the 60 to 75 before selling them as CPO's?
I just purchased a used 2016 60D last week, and during the purchase process they told me the battery was going to be unlocked and the car rebadged to a 75D. And when I showed up for delivery, that is infact what I received. So they are definitely doing it, at least for their used inventory.
Best way is to pay for and download the Tesla Power Tools app on iphone. Then sign into your account and fire it up while you're driving. Mash the pedal to the floor. IF the max power you see is ~270kW then it's still corked. Uncorked cars will show ~330kW.