Thanks. That worked just fine - I installed SME Server on the new hardware and waited until the reboot/startup before backing up the old server to USB and and taking it down - downtime was about two minutes - or it would have been, except that after restoring the backup onto the new machine, it didn't recognise the network card.
This was easy to sort out by just taking the NIC out of the old server and installing it in the new one, but what actually happened here? - I guess it could be just that the onboard network in the new server happened to be an unsupported one, or is it the case that the network device configuration was part of the backup?