That would work in some cases, but my drives are all 7+ years old so I replaced them with newer, larger ones. The new server can't fit all of my old and new drives at the same time.
In case someone searches for and finds this later, a solution suggested elsewhere is to just get a trial key for the new srever, then after the old server is done copying, shut it down, copy the config folder (other than the key) from the new server/trial usb keychain to the old server usb key, and then that should work going forward. The license is tied just to the physical USB drive, not to the drive+server.