Hi matveevn,
As far as I'm concerned, I'll appreciate to set up a totally new WSUS server 2012 if I'm not such familiar with the WSUS migration process.
Add WSUS role on 2012 server, check needed products and classifications, then sync from Microsoft Update.
On WSUS policy for WSUS clients, I'll redirect the WSUS server to the new one. Here we need to notice that, WSUS 2012 use port 8530, in the WSUS server URL, we need to append the WSUS port, for example, WSUS server:
http://newwsusserver:8530.
Related documents:
Deploy Windows Server Update Services in Your Organization:
https://technet.microsoft.com/en-us/library/hh852340.aspx
Best Regards,
Anne
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