I've been all over, even looking to custom bios flash. Seems HP has locked out any possible user upgrades to the point where even some Intel cpu's are not usable. It's a form of proprietary, if you want better you gotta buy better, you can't do better by yourself. The only possible way around it would be to find a model that uses the Leon board, h61 chipset with the same Ivy cpu you want and flash that bios. If that doesn't work, however, you'd be totally out of luck and the whole pc would be useless as I can't even find an original bios, nevermind an update. So, if you are ready to throw the pc away, it couldn't hurt, but if you plan on keeping it, I'd not try that route. It would be an absolute last ditch, all or nothing, hail Mary deal.
HP basically designed those AIO pc's to be used, then thrown away when no longer viable.