[BadBadJellyBean]

I don't think that would work. The IPv6 Notation is only the human representation. Inside the package it is a 128 bit integer. IPv4 addresses can already be put there. The problem is that everyone still needs to support a package with 128 bit addresses instead of 32 bit. There is simply no space in the IPv4 package format for the 128 bit addresses. Or anything bigger than 32 bit.

[Ekaros]

Well there is the options... But that means again every single router and host would need to support it on the path for it to be any use. So new protocol is much simpler and allows fixing other things as well.