When you take your car to a dealership to have keys programmed to a new ECM, and they take the old ECM out, do they "deprogram" the keys from your old ECM?

I bought a used ECM off ebay and had it programmed, its not exactly the right one so I was considering just driving with it and sending the other one off to be repaired.

I tried to put the old ECM back in my car and it wont start it anymore, as if it were from a different car..

Does the ECM have a battery or capacitor that discharges when unplugged, or does it sound like the techs removed my keys from the ECM when they programmed the new one?