Hi tai chan,
When you do a simple install the installation wizard will not detect the remote web client. It will only upgrade the components which are present on the particular system.
You can unregister the web client. Or power it off and then install a new web client on the vCenter server.
So to summarize, you can install a new web client on vCenter server and also upgrade the other components using individual installers.
Let me know if it cleared your doubt.