Nemesis returned Worf to the Enterprise
When Star Trek: Deep Space Nine ended in 1999, Lt. Commander Worf, played by Michael Dorn, accepted a position as the Federation’s ambassador to the Klingon Empire. He would have been working closely with his friend, Martok, who was the new Chancellor of the Klingon High Council. It was the perfect match for a pair of gruff Klingons who’d helped save the space station and protected the Bajoran wormhole. But it was also a match that didn’t last very long as Star Trek: Nemesis reunited him with his other cast members.
Worf returned to The Next Generation cast in Nemesis not only as a member of Starfleet but a crew member aboard the U.S.S. Enterprise. His time as an ambassador had evaporated, and the reason behind his sudden reappearance in Starfleet wasn’t revealed onscreen.
Nemesis needed the full cast of The Next Generation
According to Rick Berman’s quote in The Fifty-Year Mission The Next Twenty-Five Years, he, John Logan, and Brent Spiner knew that the storyline of Nemesis was going to break up the group (the cast). With Spiner wanting to be killed in the story because he felt he was too old to continue playing Data, it was understandable that the producers would want every cast member back before they lost one, especially in the event another movie would be produced.
It would have been difficult to incorporate Worf into the movie if he still retained the title of Ambassador. A Federation Ambassador wouldn’t have been fighting alongside the crew members of the Enterprise as that role was all about diplomacy and maintaining a good relationship between the Klingons and the Federation.
But there was an out provided for the writers/producers of Nemesis had they chosen to utilize it. An ambassador was appointed by the President of the United Federation of Planets, and when a new president took over that position, It was customary for an ambassador to resign. That gave the new president the chance to reappoint the ambassador or appoint a new one altogether. So Worf could have returned to Starfleet with the simple explanation that he had resigned which is how the follow-up Star Trek novels explained his departure from the position. Unfortunately, though, that route wasn’t chosen, and fans just had to accept that Worf was back onboard the Enterprise for whatever reason.