Still have a Surface RT that you relegated to the closet after you realized that it couldn't run "real" Windows apps? Don't send that tablet to the recyclers just yet -- it might just get you some credit toward a much more useful model. Microsoft is running a US promotion that will let you trade in any Surface by March 8th and get cash toward the purchase of a Surface Pro 3. So long as the outgoing device is in good working order, you'll get as much as $650 off a shiny new slate -- if it's one of the more recent ones.