I'm moving WSUS from 2008R2 to 2012R2, & am now using the procedure Anne suggested to me: https://social.technet.microsoft.com/Forums/windowsserver/en-US/098abfc4-6ed6-4700-bda5-f83acdc1cf71/move-wsus-from-8r2-to-12r2-sql-connect-error?forum=winserverwsus
The procedure is elaborated a little here: https://thwack.solarwinds.com/thread/64229
12R2 is set as a Replica Downstream server & it has succeeded in syncing with the upstream 8R2 WSUS. So 12R2 WSUS has thousands of Updates, & lists our five Computer groups - but All Computers & also our five Computer groups, are not populated, we have "0 computers". I haven't yet redone the GPO to point clients to the new server, but I still would have thought that as a "Replica" the new one would have picked up the computer details.
Thanks