Why?

Possibly because the producers or her agent realized how popular she was on the show.

But, it also wasn't uncommon for the last person billed in the credits on sitcoms back then to get an "And so-and-so as So-and-so" credit.

Possibly because the producers or her agent realized how popular she was on the show.

But, it also wasn't uncommon for the last person billed in the credits to get an "And so-and-so as So-and-so" credit.

I go with the second option. On DS, Mary Ann Monley (2nd Maggie) was billed last in the opening with the as credit, but she wasn't popular, probably the most "hated" character on that show (along with Sam).

I read the JumpTheShark website years ago, and Sam was the one who was really unpopular with audiences; Dixie/Mary Ann as Maggie were more "meh".

It seemed like the popularity of the Jo character blew up Nancy Mckeon's head somewhat.

It seemed in season 2 they were billing the girls by seniority. Nancy was the newest member of the cast, so she was listed last. When it became clear that she was a big reason the show had become a success, it didn't make as much sense to list her last after the other girls, but it would have been rude (and perhaps a violation of their contracts) to change the billing order, so adding the "and" seems like a fair compromise. On other shows, that sort of billing was used when you had two people worthy of top billing so you gave one of them a special credit at the end to make them stand out. I wouldn't be surprised if Nancy's agent was involved in the decision as well.