As I understand, you want to know if the Slingbox Upgrade Program works only through the web site or also at some retailers.
At this point, I can tell you for sure that the Slingbox Upgrade Program works only at www.slingbox.com. This program offers you the possibility to upgrade your legacy Slingbox model to the next generation of Slingbox models (Slingbox SOLO and Slingbox PRO-HD) and you will get a $50 off. The following link will provide you with all the related information:
If any retailer offers you a similar deal, you can take advantage of it in order to upgrade your current Slingbox model.