Once they've dated Fonzie, they never go back to dating Richie, do they? ;DDave Andrews said:Two more, from Happy Days:
Lynda Goodfriend played Ralph's girlfriend in one episode before becoming Lori Beth, Richie's girlfriend.
Ironically, Linda Purl played Richie's girlfriend Gloria way before becoming Fonzie's girlfriend Ashley.
source: http://www.imdb.com/title/tt0070992/trivia
I'm guessing that both of these earlier examples came from the first season (the "Chuck Cunningham" season!) of the show, right? I remember the earlier appearance of Purl, but not of Goodfriend.