"Just extend the address size" was certainly one of the options. Sure, it's still a change, but the point is: After this change, both protocols could have worked side-by-side. Devices that only supported IPv4, no problem, they send 32-bits. Devices that supported IPv6-as-it-could-have been would simply have zero-padded those 32-bits to match the new protocol. Talking to old devices, the zero-padding gets dropped.

That would have saved a lot of pain.