The mileage shown on the display is stored in the BCM. Unless you think you will run into issues with emissions testing, I would be more concerned as to whether or not the replacement ECM comes from a vehicle that had the sentry key immobilizer. You can tell by whether or not the other vehicle has a black (non-chipped) or gray (chipped key). If both vehicles are non-chipped, it's a direct swap. If yours currently has a chipped key and the other one is non-chipped then you can either disconnect the skim module on your car and not be required to have chipped keys or leave it connected and the ECM from the non-chipped key vehicle will automatically program itself to require the same chipped key that your vehicle currently takes. If BOTH vehicles take chipped keys, then you will need the chipped keys, ECM, and skim module from the other vehicle (the chipped key from the other vehicle would have to be attached near the ignition cylinder so that the skim module can read the chip while your old key is in the ignition cylinder). Or you could have new chipped keys cut and program them to the new ECM.