UA offers free ‘same day confirmed’ changes for Elite members (any status) which allows you to pick any flight in your booking class within 24 hours of your ticketed flights. You can take a non-stop and rebook it to a connecting flight for free. You can change a connecting flight after your first segment is marked as flown (e.g. from in-flight Wi-Fi) to add yet another connection. In essence you can keep doing that (within certain limits such as overnight layovers etc that won’t work in most cases) to add more segments to your trip - works pretty well if you have some flexibility, but this also has risks. Say there is a major irop somewhere in the network you might find most flights (near) sold out and thus SDC not being an option.