The sidegrade pricing has, built into it, the ability to sidegrade to Vue 4 Professional from Vue d'Esprit so that you get full credit for your purchase of Vue d'Esprit. Its based on the full price of Vue 4 Professional. Vue 4 Professional has an introductory price of $499, which is scheduled to switch to its regular price of $599. That's where the $100 shift takes place. There is still a $200 difference between the upgrade price and the full price. Best regards, Lynn Fredricks e-on software