I was recently asked by a couple of friends, is it required for me to update my ConfigMgr clients after upgrading my site? Intuitively I knew the answer and the why, but they need “proof” or documentation from Microsoft that stated it as such. It was a bit of a needle-in-the-haystack to find, but the following reference on TechNet illustrates the case.
From the guide for Planning to Upgrade System Center 2012 Configuration Manager, in the upgrade checklist, the last step is to Upgrade Clients.
“After you upgrade a primary site, plan to upgrade clients that are assigned to that site. Although a Configuration Manager primary site or secondary site can support communication from clients that have a lower service pack version (including clients that run Configuration Manager SP1 talking to a site that runs System Center 2012 R2 Configuration Manager), this communication should be a temporary configuration. Clients that run a previous service pack version of Configuration Manager cannot use the new functionality that is available with the new version of Configuration Manager.”