You can do it but it's unsupported. But I've gone back minor revision releases on several occasions. The only way to do it is with a source buildworld/kernel and installworld/kernel over the top. But whilst it would probably work fine, there is a chance it will break. So have backups, be prepared to have to reinstall it.
Do you *really* need 9.1? What you have installed is going to be virtually identical anyway, just with a few bug fixes here and there.
