Here's the latest that I've been able to discover. They originally planned the Atlantis idea to be the feature film, so they could "evolve" the stargate (producer/creator's wording). Instead, since the show is still very popular but the original cast is ready to move on, they've decided to turn the feature film script into the S7 finalle/S8 premier, that will in turn launch the new series, where the original cast will make guest appearances from time to time after SG-1 is over.
Only one cast member is scripted to die this season, and I've heard who it is (not gonna say :devil
, although I'm not sure why this person is leaving -- could be the actor, could be other reasons to move the spin-off forward. RDA will have a rank change though it's unclear whether he will take over the command of the SGC -- there's rumor that either he or Hammond will be re-assigned to command the Prometheus (sp?). We do know that RDA's roll will be diminished still in S8 because he wants more time with his 5yr old daughter and wants to do some projects "close to his heart." So the series is acommodating him on that. It's unclear whether SG-1 will just be a 3 man team during S8 or whether they'll bring in someone to join them -- such as Corin Nemec's character from S6 or someone new. Carter's new boyfriend, David, will be returning but apparently only in a guest starring role on occasion.
I suppose that there is still always a possibility that they might one day go back to the big screen with SG-1, anything is possible. If they ever did it would be with the series cast, not the original film's cast. That is a fact...no question. In fact, Kurt Russell has visited the set of SG-1 before, and loved what RDA and the others had done with the character and story.
The new series will, as I said "evolve" the Stargate. There will be guest appearances by the OC, but it will not be a military lead post like the SGC. It will be an international group of scientists, with some military support. The new gate found in Atlantis, was built by the Ancients themselves, and will take the crew to a planet in the Pegasus Galaxy, where they will discover a more primitive group of humans (how primitive they won't say -- prehistoric, or like 19th or 20th century...who knows), who are under threat from a newer and meaner enemy that is supposed to put the Gou'aold to shame. The same people behind the wonderful writing, production design, FX, and directing on SG-1 will be on Atlantis as well, so it has a great start already. They're smart enough to start the new show with one more season to go on SG-1, so the audience has time to adjust to a new set of characters before they loose the old ones.
While I will miss the cast terribly...I am very excited about this new show, and with the people behind it that are...I'm sure it will be wonderful.
So anyway, that's about it from my research. The feature film script will be the S7 finalle and the S8 premier. Look at it this way, should be one hell of a kick off to Atlantis considering that it was supposed to be their big screen blast off.
God, I can't wait! I'm almost as excited as I am about the new B5 project!
Peace! CE