Additionally to Scott Duffy's suggestion - Google Analytics Goal for the free downlaod, you can give it a unique id.
So, you get a connection between download and purchase of the upgrade. If you use Analytics well, you propably can extract the paths paying customers took on your webpage (coming from landing page, adwords, generic url).
Google Analytics (free) has a feature called goal tracking. So if you wanted to track people as they progress from your landing page, through subsequent pages, to registration or purchase, I would suggest that. Other companies provide paid solutions for analytics and tracking, but start with the free version from Google and work your way up to that if you feel you need one day.