It just won't work, until the BIOS is updated. And if you try to update the BIOS with a first gen Athlon or low end CPU, there's a good chance it won't recognize the newer CPU, because the microcode changes required on many of the B350 and X370 boards drops support for the older low end models to make room in the BIOS ROM for the microcode needed for the newer CPUs, and it CAN'T if you have an older CPU installed that meets that criteria while trying to flash it. On those type systems you probably, so far, from what I've seen, NEED to have a board with the BIOS Flashback feature, and that board doesn't have that.
So, it's a roll of the dice IMO. It could work, but I've seen three cases so far where it wouldn't, but those all had Athlons, not Ryzen 3, 5 or 7's installed. If you have a 3, 5 or 7 currently, then a normal flash to a version that gives support will probably work fine.