1.x.x.x.x will be for legacy stuff. If anything is trying to route to only 4 octlets, routers add a 1. in front of it. Bam, legacy solved.
edit: I realised people don't see the big picture here.
Step 1) We collectively engineer and deploy ipv8, along with RFCs explaining how future expansion works
2) We put all sorts of preparatory work, conversation into the eventual expansion into 6 octlets, from 5. We'll need that once we colonize multiple systems.
3) The RFC will state that "new planetary systems get space on 5th oclet, 1.200.x.x.x.x to 1.250.x.x.x.x.
4) As systems grow to need more than an ipv4 range, we transition them to their own 6th oclet. It's easy for them, because they can retain legacy 5th octlet headers.
5) We then collectively freeze our brains, so that in the future, once interplanetary, faster than light communication exists, we are thawed to deal with the 5 to 6 oclet transition!
And we all become immortal.
But no, that's fine, think small, stay non-immortal with your ipv6 Flanders!
Internally, an IP is just a 32bit number. Adding another octet is adding another 8bits to the number. IPv6 merely expands that number to a 128bit number.
If you do the math of say converting 8.8.8.8 to 8256^3 + 8256^2 + 8256^1 + 8256^0 and ping the resulting number, it will work.
If that's all IPv6 did that would have been one thing. ipv6 changed tons of things, from arp to nat, dhcp to dns.
Only for unidirectional UDP.
How do you think the other side would be able to respond if it's unaware of the 5th octlet? Basically every protocol in existence requires two-way communication and there's no backward compatible way of providing that.