[paperplatter]

Not so. IPv4->6 removes all existing v4 address blocks and redoes the addressing scheme. Those changes weren't necessary for expanding the address space. This implies that day 1 of using ipv6, all your addresses are different (and way longer), all your routes change, DNS DHCP etc all need to be swapped out, and bans/reputation are all reset with no clear replacement. And there were a bunch of smaller changes / new features.

Whatever you do to get more addresses, it will look similar in the end, but the steps could've been very different.

[kemotep]

How would a computer with an expanded address scheme communicate with another device with an unexpanded IPV4 address? How would that client get its response back?

How would a router know what to do with it without updating its software?

At the end of the day, you are talking about using a different than IPV4 address scheme and using a different protocol than IPV4. Everything in the stack will need to be updated. Every hop on the route, every single piece of software that will interact with the network address. Backwards compatibility still requires everything but the older device to be updated.

[paperplatter]

Yes, everything has to be updated eventually, but going forward doesn't have to be this hard. A network and its hosts could start supporting ipv6 without changing anything else. Same addr and routes as before, same NAT, and no DNS6/DHCP6/etc, so very low effort and risk to turn it on. If a peer only supports v4, talk v4 to it for now.

Then once there's sufficient v6 adoption, you can disable v4 entirely and start using /40, /48, etc..