Thanks for all of the notes and suggestions. Its great to be able to put these questions out there and get such thoughtful responses! I thought I'd let you know the outcome of this one.
We're at the stage now where final decisions are being made on how to proceed with this, and we're going to go with a fairly informative and interactive approach. We'll have information about the advanced product available via splash screens during the standard installation process, and we'll have a link to upgrade information and purchase procedure (via a HTML page) under the help menu. I'm also planning to include some tool tip type info at strategic points during the user's interaction with the product to tell them about relevant advanced features. These tips will of course be able to be turned off.
Lastly, we're going to produce a set of streamlined documentation for the standard product, which will contain information about advanced features and upgrade procedures, either in the preface or in an appendix with cross-references throughout the relevant document sections.