NASA and Boeing Co. agreed to turn the initial test flight of the company’s commercial crewed capsule into an operational mission, one of several recent signs officials are hedging their bets on when U.S. spacecraft will start regularly ferrying astronauts to the international space station.

Thursday’s disclosure by the National Aeronautics and Space Administration suggests a previously planned two-person flight, slated for November 2018, is now likely to occur in 2019 or 2020 and would likely carry one additional crew member...