I'm not Ken, but I think they do need to be flashed.
My belief, somewhat factually based, is that the core unit lacks a "personality module" which essentially keys the computer to work in a particular application. With different vehicles having two or four O2 sensors, different operating voltages and a myriad of variables the keying to your vin zeroes the computer into a place where it is then compatible with your vehicle. Its that way with the TCM, and I have had to pay for a GM flash before installing a new TCM. I think it is the same with a BCM, so logic suggests the same regiment applies to an ECM
