Migration is not going to happen overnight and it’s going to require simultaneous support for both IPv4 and IPv6 until both sides of the equation are ready. 

Making the switch from IPv4 to IPv6 is not a task anyone with any significant investment in infrastructure wants to undertake. The reliance on IP addresses of infrastructure to control, secure, route, and track everything from simple network housekeeping to complying with complex governmental regulations makes it difficult to simply “flick a switch” and move from the old form of addressing (IPv4) to the new (IPv6). This reliance is spread up and down the network stack, and spans not only infrastructure but the very processes that keep data centers running smoothly. Firewall rules, ACLs, scripts that automate mundane tasks, routing from layer 2 to layer 7, and application architecture are likely to communicate using IPv4 addresses. Clients, too, may not be ready depending on their age and operating system, which makes a simple “cut over” strategy impossible or, at best, fraught with the potential for techncial support and business challenges.

More Here