I was always told you can't, you can go up but not down, although that may be a little different context than what you are trying to do as I think that was meant for upgrade/downgrades... Go get a SATA 128GB SSD or any SATA 80GB+ 2.5 HDD and pop it in there and do a clean install. You will need to restore the 8.0 database as a 9.0 database cannot be migrated backwards.