Frequently, a new version will first install a new version of the bootloader, and it won't flash software that's older, nor it will not allow you to re-flash the older bootloader. That still doesn't mean it will work, though. We've seen other companies offer a way to unlock bootloaders and have archives of older versions for certain phones that were sold unlocked. If you have a Pixel or a phone from another company that is willing to let you unlock your phone's bootloader and will provide you with a catalog of all the various versions of Android that apply to it, rolling back might be reasonably easy.
