Yes, it is possible to flash another firmware. There's no need for the TECH2.
Thanks for your reply. I actually have tried flashing my MY01 Z22se ECU with MY03 w/o EGR firmware( 09391273 Ver.8--->12586913 Ver.1). Untortunatley, it won't work. Firstly, the ECU's firmware version is tied to car's VIN (
check here). It is not possible to have my MY01 to pull MY03 firmware out of GM's SPS program. Secondly, I managed to find a VIN (
from here) with similiar configurations to my real VIN and finally pulled out the MY03 firmware then flashed it into ECU. But it is not possible to get into the Engine Diagnosis menu via TECH2. My TECH2 simply replied there is no applicable management system to run. I tried two times with different VIN and all failed. However, if I tried flashing the ECU with MY03 with EGR version of firmware(12210443), I can access the Engine Diagnosis menu. So strange!
So I think for pre-MY2003 Z22Se engines, disable EGR by flashing GM's factory no EGR firmware is questionable. I only have Carprog(can do partial ECU dump but no writing) and TECH2 here, so I guess my experiment is over.