It appears to not be a supported move.
https://learn.microsoft.com/en-us/previous-versions/windows/it-pro/windows-server-2012-R2-and-2012/dn303416(v=ws.11)#upgrading-previous-retail-versions-of-windows-server-to-windows-server-2012-r2
The simplest / safest solution would be to export / copy off the virtual machines to removeable, clean install the operating system, patch fully, add hyper-v role, import virtual machines.
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