I can think of quite a few, but I’ll only mention some so that I don’t hog the spotlight.

John Ratzenberger as Cliff Clavin - During the first season of Cheers, John was listed as a guest star. He appeared in 21 of the 22 episodes before joining the cast in the second season.

Jim Rash as Dean Pelton - For the first two (of six) seasons of Community, Rash was a recurring character. However, in the third season he joined the cast officially.

Victoria Dillard as Janelle Cooper - She was recurring during the first season of Spin City. For seasons two through four, she was an official cast member. She left the series along with Michael J. Fox.

Danielle Fishel as Topanga Lawrence - She was a recurring guest star in the first season of Boy Meets World and joined the cast officially in the second season.

Jaleel White as Steve Urkel - First appearing in episode 12 of the debut season of Family Matters, Jaleel White remained a recurring guest star for the rest of season one. He joined the cast (and subsequently took over the series) in season two.

One I recall is Ken Kercheval's Cliff Barnes recurring in the first two seasons of original-recipe CBS Dallas; he became a regular starting with the third go (1979-80).

Monica Horan - 8 seasons recurring and 1 season as a regular on Everybody Loves Raymond

On Newhart,

Michael
Larry & the Darryls

appeared occasionally before becoming main characters.

Grandma Romano on One day at a time

Roseanne and Jackie's Mother Bev on Roseanne

Richard Kline-Three's Company

During, I think it was, the 8th season of One Day At A Time, Shelley Fabares was part of the opening cast credits.

(Francine was a great part for her--really the polar opposite of Mary Stone).

Nice question, Adam.

I believe the characters of Levitt and Inspector Lugar from "Barney Miller" would qualify here.

what about Frasier on CHEERS? He was only on the show sometimes, then became a regular when Shelley Long left.

The character Gibby on iCarly guest starred on only a few episodes in the first and second seasons before becoming a regular in the later seasons and appearing in every episode after that.

Ed.

The character of Fonzie on Happy Days was definitely a minor character (not sure about recurring) in the beginning of the series. He eventually became the lead of the show (after Richie) and surpassed most everyone in popularity.

Markie Post - Night Court

what about Frasier on CHEERS? He was only on the show sometimes, then became a regular when Shelley Long left.

I remembered him, but I couldn't remember what season he joined the cast. I know he was recurring beginning with season three but I forget when he appeared in the opening credits, season five or six. So I just decided to let someone else mention it. :)

The character of Fonzie on Happy Days was definitely a minor character (not sure about recurring) in the beginning of the series. He eventually became the lead of the show (after Richie) and surpassed most everyone in popularity.

Henry Winkler appeared in all 255 episodes so Fonzie was definitely not a recurring character.

Full House actually has two known examples of this:

Lori Loughlin as Rebecca Donaldson - She was recurring in the second season of Full House. Starting in season three, she was an official cast member.

Andrea Barber as Kimmy Gibbler - For the first four (of eight) seasons of Full House, she was a recurring character. However, in the fifth season, she joined the cast officially.

Barry Livingston - My Three Sons - recurring for a few seasons as Ernie, then joined the cast in Season 6 (replacing Tim Considine).