If You just delete IMMO from ecu, the car will start but You will have the annoying blinking immo light on the cluster. Just did this to an A6 a couple of moths ago. Swapped newer IMMO3 cluster to replace a broken IMMO2 cluster, adapted with VCDS. The ECU is still the original but all in all, it's the same operation.
Only if you are using a unpaired key. Just swapping ecus wont do that