And it is. That's exactly how deploying v6 works. In fact, even the packet format stays the same, which means you don't lose the ability to talk to peers that require the old format -- people would do more than hesitate if that wasn't the case.

But you understand that after ISPs get short on addresses and start to hand out /64s, it's not going to be exactly the same afterwards, right? You'll have to actually use that /64 and the updated DNS and stuff.

Because we hit that point twenty years ago. We're long past the "everything looks and works in exactly the same way (so only the 32-bits of addresses in v4 work)" stage and deep into the "ISPs hand out /64s" stage. There was a point where v6 deployment just meant that you turned it on and nothing else at all changed, but at this late point in the game it also involves using those new addresses. We already took the approach you're asking for here, we just didn't stop at the beginning of it.

So there was a time when if I enabled ipv6 on my router and PC, it meant I'm still behind NAT, my public and private IP addresses are the same (except mapped onto v6), my DNS is the same, and all packets take the same routes?

I'm too young to remember, but I've dealt with old routers and PCs, don't recall that ever being the default. If it was, they took the second step too early.

Btw, there's also a difference between dividing up the existing blocks vs handing out new ones.

To clarify, my private ip is 192.168.1.3, public 1.2.3.4, and going to v6 I get ::FFFF:192.168.1.3 and ::FFFF:1.2.3.4, and no additional addresses