Comment by dnlrn

Microcode is not written to the CPU, it gets loaded on every boot. This can happen during the BIOS POST, during the OS bootloader or even while the OS is booting. Therefore, yes its possible to run older microcode (at least on Linux), since you just have to not write the newer version on boot. If the BIOS contains the new microcode, you can flash the previous version of the BIOS.