Comment by dmitrygr

Yes, but let's not pretend that IPv6 was just IPv4 with a larger addr size. A lot of other unnecessary cruft got changed. That is kinda the issue. Add an extra octet to IPv4, call it IPv5. Done. DHCP (as is) can already handle variable-length addresses so it'll work unmodified for 5-byte addrs, for example. Same for ARP.