Premiere Elements is a complete installation. You could ask Adobe if there is an upgrade price for but realistically you should just buy the boxed product and install from the correct DVD.
Insanity is hereditary, you get it from your children
Welcome to the forum.
As Neale points out, "upgrading" is but a price break, and can be found here: http://www.adobe.com/products/premiere-elements.html
The full Premiere Elements and Photoshop Elements suite is often the best "deal," and also some retailers may well have the full product(s) at even lower prices, than Adobe offers, under the name of an "upgrade."