Now, I've never worked at a video game store so I have no idea what's involved or how true this is... But about a month ago the clerk at my local EB Games tried to sell me on purchasing the new Halo Green special edition XBox. I told I already had an XBox. He informed me that I could trade the old one in for $80... and that they could transfer the memory from my old HD to the new one.
Hmmm. Then there is a way to do it, and apparently without ruining your warranty, unless this guy was lying. You may want to go to a video game store and ask them about it.