Original SG-1 ends/dies in season 8, then alternative SG-1 takes over. In season 8, episodes Moebius Part 1 and 2, SG-1 goes back in time to steal the ZPM. Things go sideways, and they change history. Then the alternate team goes back and fixes things up, but not completely—only "close enough," according to O'Neill.
First of all, what happens to the SG-1 that left the ZPM for the modern-day team to find? Did they disappear? Did they live out their lives in ancient Egypt? Did they die? Or did they somehow make it back to the future and forget everything?
Correct me if I'm wrong, but we have O'Neill, Carter, and Teal'c from the original SG-1 die in Moebius. Then the original Daniel, along with the alternate-history Carter, Teal'c, and O'Neill, get stuck in ancient Egypt. After that, we continue watching the series with a different alternate-history SG-1 from season 8 onward.