“The Changeling” is episode nineteen of season six of Stargate SG-1.
Teal’c is having rather odd visions. He’s having visions of having nightmares and of no longer being a Jaffa, but of being a fireman, with still the same comrades. As a fireman, he is also seeing other people he knows, but in different situations. The people include dead or ascended ones.
An odd episode for much of it, with most of it spent as a fireman, interspersed with brief bits where the people in the, hopefully real, world try to work out what is wrong with Teal’c. And then the real world starts getting mixed up. Until finally what is truly happening is shown.
As this season is nearing its end, I think I did see some of season seven as well.