On NCIS Ducky's assistant Jimmy Palmer (Brian Dietzen) became a regular after starting as recurring. On Happy Days Fonzie (Henry Winkler) was only intended for a few episodes but became a regular and eventually received top billing when Ron Howard left. Dr. Frasier Crane was only intended to be a 6 episode arc on Cheers but of course was made a regular and Kelsey Grammer won Emmys portraying him on three different shows.

Sitcoms and dramas only, no reality

do old radio shows count? I hope so because I know of 2 on radio shows: Mary Livingston and Rochester on THE JACK BENNY SHOW. once on the show they had a storyline involving a "win a date with Jack Benny" contest. The "winner" was a girl named Mary Livingston. But, they needed someone to play her. Jack suggested his wife Sadie. She agreed, and the character was only supposed to be in a couple episodes but she became so popular they made her a regular as Jacks girlfriend. So many people thought her name was really Mary Livingston that she changed it to that; legally.

And Rochester first guest-starred in an episode as a railroad porter when Jack was taking a trip. Then in another episode, he came to visit Jack once. The character became so popular they made him a series regular and he decided to quit the railroad and go to work for Jack as his valet.

Larry and his brothers Darryl on Newhart!

Klinger on MASH

on Beverly Hills, 90210:

Claire Arnold was first a guest star and then recurring character in the early seasons before becoming a series regular in the college years.

Dylan McKay was a guest star before being named a regular.

on Melrose Place, Amanda Woodward was originally a recurring character who was supposed last a few episodes, but she was so popular she stayed for the series entire run, and became the unequivocal lead of the show.

I didn't watch it, but does Urkel(Family Matters) fit this?

Little Ricky on I Love Lucy was a recurring character as a baby but became a series regular when 5 year old Keith Thibodeaux was hired to play him as a toddler.

IIRC, Ken Kercheval was a recurring cast member (and Cliff Barnes a recurring character) in the first two seasons of Dallas before going regular in 1979.

Seven-Of-Nine on Star Trek: Voyager.

You're right opus, Steve Urkel was only supposed to be on the one episode but was so popular he became a regular. Which totally ruined the show for me. :P

I think Patton Oswalt's character Spence was also only a recurring character on The King Of Queens before becoming a regular. The same thing with the character of Holly, at least for a couple seasons.

And I think the character of Skippy on Family Ties who had a crush on Mallory was at first a recurring character before he was made a regular.

Ed.

Seven-Of-Nine on Star Trek: Voyager.
I think you're wrong; she was always intended to be a regular.

Sue Ann Nivens and Georgette Franklin (The Mary Tyler Moore Show)

A Different World - Ron (Darryl M. Bell) and Walter Oakes (Sinbad) both started off as recurring characters in Season 1 before they were promoted to regulars and even receiving opening credits billing.

The Rockford Files - thanks to the newly restored HD prints Me-TV is now using, Sgt. Dennis Becker (Joe Santos) was a recurring character in S2 before he got bumped to series regular and eventually receiving opening credits billing from about S3 or so onward.

Jim Rash as Dean Pelton on "Community." He was recurring in the first two seasons and officially joined the cast in season three.

Farrash Forke as Alex Lambert on "Wings." She was recurring in season four and a cast member in season five (and then left the show after that).

Clea Lewis as Audrey Penney on "Ellen." She appeared in one episode of the first season and then was recurring in the second season before joing the cast officially season three.

Clea Lewis as Audrey Penney on "Ellen." She appeared in one episode of the first season and then was recurring in the second season before joing the cast officially season three.


And in her first appearance she was a very different character; for one she didn't wear all Pink, she had a husband and she was not the same upbeat character. That all developed later.

My Three Sons: Chip's friend Ernie Thompson (Barry Livingston) is adopted, becomes Ernie Douglas, and receives billing -- replacing Mike Douglas (Tim Considine).

Lassie: Young Timmy (Jon Provost) begins making recurring appearances with the family of Ellen Miller (Jan Clayton), eventually replacing Jeff (Tom Rettig) with star billing -- following his adoption by the Martins.