Application Transition

Well, on your original question, you'll need to determine how you want to identify the users to be moved. The obvious options are IP address and cookies. If you go with IP addresses, your subnets should be carefully chosen to avoid splitting big dynamic pools. (So for example, if you decide RoadRunner customers in Seattle should get the new site, then you would want to do some due diligence to make sure you move the whole dynamic block, otherwise you might get people flipping between old and new sites when their local IP is refreshed.) If you go with cookies, you can be more certain that particular users will stay where they're put, as long as they don't clear their cookies.

Another option, if your application architecture supports it, would be to design a shared static login page and redirect users by username. This would avoid the above problems but might or might not even be possible, depending how your system works.