Endgame. Actors are locked in for at least a couple of films with plans to develop all main characters in the long run. That said, very early on in its history, the franchise had to switch the actor portraying a key character when Don Cheadle replaced Terrence Howard as Tony Stark/Iron Man's (Robert Downey Jr.) trusted friend from the military.
It was initially the intention of bringing back Terrence Howard as Rhodey in Iron Man 2 after an explicit tease of his character assuming the character's superhero persona in the Iron Man sequel. This never came to fruition as Cheadle took over the role in Iron Man 2, getting the opportunity to put on the War Machine armor — something that his predecessor never got any chance to do. No official reason has been given by Marvel Studios regarding the matter, although some of what went down behind-the-scenes made its way online over the years. While Kevin Feige and his team have remained silent regarding this recasting, they, however, found a subtle way to address the issue in Iron Man 2.
Loosely inspired by the "Demon In A Bottle" comic book narrative, Iron Man 2 saw Tony publicly deal with the repercussions of his decision to confirm his superhero identity, while privately battling for his life as he suffered from Palladium poisoning. As the government attempts to acquire his Iron Man technology, the genius, billionaire finds himself in the middle of a Senate hearing. He initially coped well in justifying his refusal to share his creation until thrown off-balance upon realizing that Rhodey was called to testify and build a case against him. Cheadle's first appearance as Rhodey comes during the hearing; as he enters the chambers, Tony tells him that he didn't expect him to be there. Rhodey's reply was short and simple, he tells his friend: "it’s me, I’m here, deal with it, let’s move on."
While Cheadle's dialog makes sense given the circumstances MCU's War Machine moving forward.
Now, sadly the genius billionaire is gone after the events of Avengers: Endgame. Among the original six MCU heroes, Iron Man is the only one with no clear replacement and there's an argument to be made that Rhodey should actually have been given the chance to fill the void that Tony left considering he'd been with Stark at his lowest points as seen in Iron Man 2. At this point, however, it's unclear what the future lies for War Machine in the franchise or even if there will be one.
