When you install Opus in evaluation mode (i.e. without then installing a purchased certificate), you can switch between Light and Pro during the installation and also via the LIcence Manager.
Once you install a purchased pro certificate, you can't switch any more on that machine (except by installing a purchased Light certificate).
Light is a cheaper subset of Pro so I can't think of any reason you'd want to use Light if you already have a licence for Pro on the same machine.