'DWTS' season 27: Olympian Mary Lou Retton and surprising 'new' choice expected to join cast (Exclusive)

The cast of Dancing With the Stars season 27 is shaping up!

Three weeks before the competition kicks off, ET has learned of two new celeb contestants joining the show -- and one who won't be.

A source tells ET that Olympian Mary Lou Retton is joining the cast. "She turned 50 this year, but is in great shape and will definitely impress people with her flexibility and choreography," the source says of the former gymnast, who won a gold medal in the individual all-around competition as well as two silver medals and two bronze medals at the 1984 summer games in Los Angeles.

While elite athletes are nothing new for DWTS (after all, last season wascomprised of all athletes), ET's source reveals that the show is trying something "new" this year, with model Alexis Ren.

See last season's pairings:

"She’s a young model who rose to fame on Instagram and has been in Maxim and Sports Illustrated. They haven’t really gone the influencer route yet, so this is new to them," the source explains. "But besides being young and attractive, she also has a very compelling story. Her mother died when she was just 17 years old and she’s struggled with an eating disorder in the past."

Sarah Michelle Gellar was also rumored to be joining the show, but according to ET's source, she's not expected to compete this season.

ET reached out to ABC, who replied that they don't comment on casting rumors. The full cast will be announced on Good Morning America next Wednesday.

Already announced as part of this season's cast is Facts of Life star Nancy McKeon, who will be partnered with Val Chmerkovskiy. See more in the video below.

Season 27 of Dancing With the Stars premieres Sept. 24 at 8 p.m. ET on ABC.