Need help from some xbox gurus

I've got a problem here. I bought a new 360 and needless to say after a week It had the 3 red rings. My last xbox had a disc drive go out and it was trashed. So I'm in a situation where I have no receipt for my new rrod 360 and my old one has been modded so I can't trade them In. So my question is, can I take the good drive out of my new 360 and drop it into my old one with the junk drive and have it work? Or will I have to reflash the new drive to work in my old xbox? And if I do have to reflash the new drive,how do you do it?--Josh