The surprising solution to this quandary that AMD have gone with is to offer a short term loan of an Athlon 200GE CPU with accompanying thermal paste and a boot kit to handle the BIOS upgrade. The plan being you throw together this temporary system, get all your BIOS ducks in a row, then pop the loaner chip out and send it back.
To get the loaner chip you have to contact AMD online warranty claims and enter ‘Boot kit required’ in the problem description field. There are some further terms and conditions described in AMD’s support FAQ here. You need to provide pictures of your motherboard and the new chip for example, and you need to have contacted the motherboard company first so that you can then explain to AMD why nothing but a loaned chip will fix the situation.
So it’s a nice thing to do for people who find themselves in a post-upgrade pickle, but this is still a bit of a mission to set up. Not to mention having to wait for AMD to okay it and then send out a loaner chip will create that kind of agonising time vortex that exists when you have new components in your possession but can’t put them to work.
The lesson here of course is to keep firmware up to date, and take care especially before an update of this nature to give everything a check for last minute updates. Also keep your old components handy, at least until you’re sure that the new parts have bedded in properly.