title: How do I upgrade without adding files with WiX?
body: <p>I'm making MSI's with WiX for our product. The product loads some optional assemblies as modules. This allows us to just drop a dll into the directory and charge a bit more!</p>
  
  <p>Now I want to make a common major upgrade MSI that includes all the possile module assemblies but only upgrades files that exist, and doesnt install missing ones. How can I achieve this?</p>