In what way would this have "Complete backwards compatibility"? Maybe read https://en.wikipedia.org/wiki/Internet_Protocol https://en.wikipedia.org/wiki/Interne

In what way would this have "Complete backwards compatibility"? Maybe read https://en.wikipedia.org/wiki/Internet_Protocol https://en.wikipedia.org/wiki/Internet_Protocol first, specifically the part where it specifies the on-the-wire format. What backwards compatible change would you actually propose there?