Stargate SG-1 followed the old "Planet/Monster of the Week" episode format that was used in Star Trek TOS/TNG and the X-Files.
While the later, like Stargate SG-1, had an overreaching arc it was mostly standalone (or apparently standalone) episodes.
The one you mentioned, The Changeling
was a very important episode for Teal'c's character development. Many standalone episodes are used to develop characters, like A Hundred Days