That’s interesting speculation, but completely wrong. In plays, actors substitute for others when one is unavailable for a given performance. TV episodes were originally seen more like plays, but with recurring elements carried on through many series. Movie actors also played different roles from one film to another and still do.

In early TV shows, it was not uncommon for one of the lead characters to have different actors come and go. George Burns’s neighbor was the one of the four main characters in each episode. On one episode, in the first season, there was an actor who played him as a substitute for one episode, another who came, went and came back, and an eventual older actor who replaced a very different younger actor. Andy Griffith had character actors who must have played a dozen or more guest roles, each as the one the the theme of the episode was based on. Two of them were Jamie Farr. The show also had two “Floyd the barber” actors. Bewitched did the same thing with recurring actors, characters, and even with a lead role.

On shows like The Twilight Zone or Alfred Hitchcock Presents, it was common for actors to come back and play different parts in different episodes, but people don’t question that any more than they question Brad Pitt playing different characters in different movies. But neither did TV audiences on episodic TV. On MASH, of course the producers knew that Colonel Potter was a general on an earlier episode, and compared to that, which actress played which nurse on a given day was a small issue.

These days, TV shows tend not to bring in the same actor to play different parts on the same show, but even recent shows like 2.5 Men had a lead actress play three or four of Charlie Harper’s girlfriends over the course of the series, including as a series regular for at least a season.

The producers made no mistake on MASH and actresses could have easily pointed out that they formerly played Nurse Able and Nurse Baker before becoming Nurse Charlie. But they accepted whatever part they were given for an episode.