The upcoming third season will feature a new lineup though actors Uhm Tae Woong and Cha Tae Hyun are expected to stick with the show. A rep of '1N2D' stated, "It's true that the producers are currently planning the cast, but nothing has been confirmed." Minho's label SM Entertainment also announced that there are no solid plans as of yet.
As for the original members Yoo Hae Jin, Sung Si Kyung, Kim Jong Min, and Lee Soo Geun, they have decided to leave the show after the current season is over.
The third season of '1N2D' is scheduled to start filming on November 22 KST, so it's likely that we'll hear about the new lineup soon!