We're running a vSphere 4.1 U3 linked-mode environment with 4 vCenters. In order to evaluate the VSC 4.2 NFS alignment capability, we'd like to upgrade one of the vCenter VSC implementations to to 4.2, but leave the rest at 4.1 until 4.2 is vetted out.
Will the VSC 4.2 plugin communicate with instances of VSC 4.1?
Are there any other caveats / risks to approaching an upgrade this way?
Currently there is no "support" for vCenter in linked mode to allow plugins to communicate through all instances of vCenter, so this is somewhat of a moot point. You'll still have to install and register one plugin per vCenter, and when accessing VSC, you'll need to ensure that you've connected directly to the vCenter and not the linked mode vCenter instance.
Therefore, in your configuration, as long as you're accessing the vCenter server directly - regardless of whether it's running VSC 4.2 or 4.1, it will work the same.