In examining the adoption of innovations, marketing researchers have generally assumed that the early adopters of one product are more likely to be early in adopting the next generation of products. However, there seems to be a missing link between buyers' first adoptions and their next-generation purchases, or upgrades. This paper tries to fill the gap by incorporating users' post-adoption behavior into the model of buyers' product upgrade behavior on the successive versions of innovations. Survey data on cell phone purchases in Korea confirm the idea that the post-adoption behavior, particularly the use of innovative features, plays a critical role in developing users' intention to upgrade.