Found this in the archives:
Date: 12/08/2008 04:21 pm
Author: Ray Borduin
The main difference between site assist and site assist pro is that site
assist pro allows you to create your own templates using Dreamweaver and has all of its templates created based off CSS Sculptor.
There is no upgrade path for updating a SA site to a SA Pro site, so they
pretty much run as independant products and you would have to recreate a
Site Assist site to create the site in SiteAssist pro.
If you really like the SiteAssist design, you could even import the old site
assist designs into site assist pro and have them as options, but we have
already created CSS versions of those pages which are more standards
compliant and follow more modern practices.