Especially in Atlas Shrugged... :X

Yet there have been changes in innumerable shows over the years - Chris Partridge, Lionel Jefferson, Christine Cagney, Miss Ellie Ewing, Becky Connor, and even Professor Dumbledore.
While continuity is generally better, different actors portraying roles is not unheard of.

Ayn Rand was actually working on adapting the story of Atlas Shrugged into a script for a TV series, as she had decided after the movie adaptation of The Fountainhead that she didn't want to work within the time constraints posed by film again (too much material had to be cut, in her opinion). Unfortunately, she passed away before she finished writing it.

Though to be honest, I personally don't think Atlas Shrugged would really have worked very well as a TV series. While there are some certain scenes and arcs which I would have liked to see in the movie (specifically, the childhood romance between Dagny and Francisco), the story is one that's built for a novel. Stretching it out into a TV series would just make the plot drag, especially if nothing was cut out. I think the audio book version is about 60 hours long, which would be either 60 full-hour episodes, or 120 half-hour episodes. When you consider that one season of a show is usually only ten or so episodes long, filming that much story would be a massive project, one which could potentially span anywhere from 6 to 12 years, and could potentially get cancelled halfway through if it didn't get good ratings (which is a high probability, considering the box office sales of the movies). Now they could certainly condense the entire story down into one season consisting of ten one-hour episodes, which would give them four more hours in which to tell the story versus what they had in the movies, but they'd still have to cut out a lot, and they'd also have to be willing to end the series after only season because there isn't anything else after it ends.

Now if they wanted to make the plot of the TV series into a sequel to the novel, rather than an adaptation of it, then they'd have a lot more freedom to craft a unique story that was specifically written with TV in mind, and thus circumvent many of the challenges associated with adapting an existing work. But this presents its own unique problem of potentially straying from the spirit of the original story, as Ayn Rand is no longer around to provide new content or ensure that the new plot is consistent with her philosophy. Ultimately, while adapting Atlas Shrugged into a TV series is a fun idea to think about, I don't think it would be at all practical.

However, if the TV series was a totally new story with a new setting and new characters, and didn't try to be Atlas Shrugged, but rather simply promoted Objectivist values, that might be much more feasible. Though at the same time, such a story wouldn't be Ayn Rand's Objectivism, but rather a new derivative of Objectivism as understood by whoever wrote the script. But that's okay. Everyone has a unique view on life, and it would be interesting to see the same philosophical construct which Ayn Rand created being projected through another person's value system.