In any case, leveraging your ADCs to help you proxy IPv6 traffic is still a valid solution that enables you to accomplish your goal while avoiding architectural changes to your infrastructure and applications.
Phase III: Connect employees to corporate resources.
No doubt your organization provides remote access solutions (such as SSL VPN) so employees can easily access IPv4-based corporate resources from outside the company. These solutions are especially important for mobile workers who provide direct support to your current IPv4-based customers.
As more of your customers move to IPv6, however, your remote access solutions must also support IPv6 so that employees can continue providing the best possible customer service. The ideal solution would let you assign new virtual IPv6 addresses to the hosting servers and then simply make the clients aware of them. This would enable employees to reach any internal resource they need, regardless of where they are located, the type of network access that's available, or the type of device they're using.
What's important to note here is that your security requirements don't change, so you want a solution that enables you to apply the same access policies to IPv6 that you currently use for IPv4. Again, the idea is to provide remote access for IPv6 with minimal administrative overhead. If your existing solution can get you there, that's always preferred to adding new point solutions.
Phase IV: Enable optimized site-to-site communication on IPv6.
While SSL VPN is great for providing secure client access, it's not the appropriate solution for connecting physically separate sites, teams or business units. Instead, a site-to-site communication solution gives remote teams a secure, high-speed tunnel through which they can connect and share resources directly without relying on a PC or workstation to have network access.
Today, few ISPs provide direct site-to-site circuits on IPv6, but the list is growing. If your current solution supports both IPv4 and IPv6, so much the better as that will likely save you time and administrative overhead. Your objective is to enable your employees to work on IPv6 the same way they do on IPv4 today. Preferably, the solution would enable you to replicate data, back up and recover systems, mirror virtualized applications -- all the same types of activities you use site-to-site communication for today on IPv4.
Phase V: Tackle legacy applications and backend systems.