Instructions
Connect the hard drive that has your "Modern Warfare II" data on it to the Xbox 360 transfer cable.
Plug the transfer cable into your Xbox 360 via the USB ports on the front of the console.
Attach the new hard drive to your Xbox 360 console, if necessary. Only systems released within the first few years of the console's release require an external hard drive.
Turn on your Xbox 360 console. A message will appear informing you that the transfer cable has been detected.
Press the "A" button on "Yes, transfer to console." A screen will appear displaying all the information contained on the hard drive.
Press the "A" on "Start" to begin. All of your data will then be transferred to your console via the transfer cable, including your saves of "Call of Duty: Modern Warfare II." This will allow you to get Prestige Level 10 even if you haven't played the game on that new console before.