This question is non-technical. It's more based on the principles of CRM and how SalesForce implements those principles:
When talking about new customers and the sales funnel it's quite clear:
Lead -> Opportunity -> Customer/Account
Let's assume I want to sell a new product (Product B) to an existing customer (who already bought Product A) (not cross selling nor upgrade)? Will, from a product view, the existing customer be a lead again? Or an opportunity?