That is the easy part - most of the core routers have supported ipv6 for decades - IIRC many are IPv6 only on the backbone. The hard part is if there is even one client that doesn't have the update you can't use the new non-legacy addresses as it can't talk to you.

Just like today, it is likely that most client will support your new address, but ISPs won't route them for you.

Yes of course I know all that. That was the whole point of the overlay first approach. i.e. Build a network that works over the existing network before adding any barriers to entry like specialized hardware requirements.