If you need to replace the motherboard, what's the point of reusing the existing socket? That said, I think I've seen this done, although the details are eluding me and web searches aren't helping; I think there was, once upon a time, a motherboard that could swap between x86 and Alpha processors with only minor reconfiguration (like, changing a jumper minor).
Deep into rumors that have been circulating in my brain for two decades now... But supposedly Slot A for the (original) K7 Athlon was very similar to the Alpha CPU bus (wikipedia says "The Athlon utilizes the Alpha 21264's EV6 bus architecture with double data rate (DDR) technology.[citation needed]"). The K7 team had some people who had worked on the Alpha at DEC, so there were lots of comparisons drawn.
Zero hardware changes for the board maker. I'm not sure it would be possible to reflash from x86 BIOS to RISC-V bios and reboot. That would be cool though. Or maybe they have a jumper to select which is expected (to switch between 2 BIOS images) and everything else is identical.
There was once a MIPS CPU that was a drop-in replacement for the 80486. It required, of course, a different BIOS.
This is one good thing OpenFirmware had going for it.
I'm still waiting for AMD or Intel to make a RISC-V chip that drops in one of their existing sockets. Yeah, you'll need to replace the BIOS and such but I'm looking to buy a new motherboard anyway.