If that's the case, the microcode update happens some time during the boot process. It isn't persistent. It's either in the firmware or the OS. If it's on Linux

If that's the case, the microcode update happens some time during the boot process. It isn't persistent. It's either in the firmware or the OS. If it's on Linux, you can disable the boot script that performs the microcode update or patch it to update an older microcode image and you're done.