In addition to physical resource planning, I want to mention about logical planning, design and migration.
As indicated in the post virtual migration might be smooth, actually If data centers close enough, migration might be almost seamless. Between the data centers interconnect link can be purchased for short amount of time and all the networking devices ( assume there is no single point of failure ) can be shifted to another data center.
Then both data center for s short amount of time will work on single point of failure but meanwhile application load can be started to migrate.
Depends on the Data center interconnect link capacity of course; application load can be carried from old data center to new one.
Out side work needs to be consider of course such as DNS , BGP announcements so on but with the help of Load balancer and proper routing design migration might be done smoothly.
There are many things need to be written to show how such as protocols,vendor specific implementation so on but this is already long comment I believe.