> downgrading is not possible

Sure it is. I've done it myself.

Microcode is stored in volatile memory on the CPU. Updates are applied on boot, every boot. "Downgrading" is as simple as not applying updates, or applying an older update.

Perhaps they meant that downgrading is not possible without a reboot.

Loading microcode on a CPU (without patching the firmware) takes effect immediately. The update is lost immediately upon reboot and must be reapplied each time.

Right, but can you load an older microcode after a newer microcode has already been loaded, without rebooting?