all the client machines needed to be present onsite
As far as I know this shouldn't be a requirement. One thing though when migrating from SBS to Standard; you'll need to move the computer accounts from SBSComputers container to Computers on the new domain controller. This could also affect group policy.
can i deploy WSUS on a server 2019 but keep the FSMO on the SBS2011 for now ?
I don't see a problem with this either.
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