Originally Posted by jsloan

Potentially, yes, if there is space available in your fare class. If you don't have checked luggage, your best chance to do so would be after you've boarded EWR-SFO, or 24 hours before your desired SFO-LAS leg -- whichever comes first, and you should try the app first. If you do have checked luggage, you'll need an agent to do it and probably won't be able to do it until you land in SFO -- and even then the airport agents may not want to do it, particularly if there's not at least an hour before your LAS flight (to pull your bag).