jmack254EVGA support has let me know that the BIOS chip itself can't be replaced on the Z370 Classified K, but they did offer to upgrade it and return to me at no charge, as they did for Jc. Given the generally weird POST behavior I'm seeing, and the strong chance that I might have other hardware that's actually defective, I'm uncomfortable waiting a week or more. I've ordered an 8th gen i5, which I will use to upgrade to current BIOS and then retry the build. I knew ahead of time that the board might not come out of the box with the BIOS needed for the 9900K, so this is not really a big issue. Thanks to all for your help and suggestions.