> backwards compatible with existing hardware/software
Absolutely not. Existing IPv4 hardware and software expects addresses to be 4 bytes long. They expect packets to have a 4 byte source address at offset 12 in the header, then a 4 byte destination address at offset 16. Everything breaks if you try to give them a header with an 8 byte source address at offset 12 and an 8 byte destination address at offset 20.
Absolutely not. Existing IPv4 hardware and software expects addresses to be 4 bytes long. They expect packets to have a 4 byte source address at offset 12 in the header, then a 4 byte destination address at offset 16. Everything breaks if you try to give them a header with an 8 byte source address at offset 12 and an 8 byte destination address at offset 20.