-> Format your pen drive.
-> Plug in Old Hard Drive into xbox, plug in formatted pen drive into xbox.
-> Transfer your profiles from the old hard drive to the pen drive.
-> Remove old hard drive and forget about it(lol j/k)... then put in the newer one and get ready to transfer the profiles back.
-> Transfer the profiles from the USB pen drive on to the new hard drive! now your profile is ready to play.
Note that you will STILL need to download some content from the marketplace again, like a map pack for COD, don't worry you won't have to pay for it since you've bought it already.