it all depends on what drive you end up with in your new 360 if it a hitachi 79fl or fk you would either need to install a passkey to get the xbox 360 key to install in you old drive or have tthe tsop lifted and read in a programmer to give you the key
if you got any other drive make ,model, in your new 360 you would be fine to get the key from it to put into your old drive but you will also have to spoof your old drive so that it reports to being the same drive that was in your new 360