alt Hacker NewsWell, I can do that with OpenWRT, no idea which prosumer devices already implement this, but it isn't rocket science: Announce the Prefix of the currently active connection, invalidate the other one. Will interrupt all your TCP connections, but they are toast anyways, most software should handle this just fine. It's quite the same as a Wifi-to-Cellular handover.
> Announce the Prefix of the currently active connection, invalidate the other one
And this doesn't actually work. Prefix deprecation is a best-effort feature that is not implemented correctly in tons of devices, including such rarely used niche operating systems as macOS. It even technically violates RFC4862 (section 5.5.3).
As usual, IETF only recently woke up to that reality: https://datatracker.ietf.org/doc/html/rfc8978
I highly recommend actually trying what I proposed. Not in a theoretical hand-wavy way, but actually setting it all up and verifying that it works. I did not pose this challenge in a "gotcha" way. I really was not able to make it work cleanly with either Mikrotik or OpenWRT routers.