* Joe Flanigan: Played the main character, Dr. John Sheppard, the leader of the Atlantis expedition.
* Torri Higginson: Played Dr. Elizabeth Weir, the original leader of the Atlantis expedition. She started as a recurring character in SG1 and became a lead in Atlantis.
* Rainbow Sun Francks: Played Lt. Colonel "S.G." (Samantha) Carter, a recurring character in Atlantis, taking on a lead role in the final season. She was a major character in SG1.
It's important to note that while Richard Dean Anderson (who played Jack O'Neill in SG1) appeared in Atlantis, he was not a leading character. He only appeared in one episode, "The Return, Part 1."
Other actors from SG1 who had recurring or guest roles in Atlantis include:
* Michael Shanks (Daniel Jackson)
* Amanda Tapping (Samantha Carter)
* Christopher Judge (Teal'c)
* Ben Browder (Cameron Mitchell)
* Claudia Black (Vala Mal Doran)
While not as numerous as the actors who moved from SG1 to Atlantis, a few actors who appeared in Atlantis later transitioned to SG1:
* David Hewlett (Rodney McKay)
* Jewel Staite (Dr. Jennifer Keller)
* Jason Momoa (Ronan Dex)
So, the main stars who transitioned from Stargate SG1 to Stargate Atlantis were Joe Flanigan, Torri Higginson, and Rainbow Sun Francks.