Sounds like they always had 16:9 footage, and just cut it down to 4:3 for the original broadcast.

All episodes were filmed in 16:9 wide-screen, although Stargate SG-1 was broadcast in 4:3 aspect ratio in its first years.[40] The transition to the broadcast of episodes in the wider 16:9 ratio gave directors more freedom in frame composition.[41] The first three seasons of Stargate SG-1 were filmed on 16 mm film, notwithstanding scenes involving visual effects that had always been shot on 35 mm film for various technical reasons. After a test run with the Season 3 finale, “Nemesis”, Stargate SG-1 switched to 35 mm film for all purposes at the beginning of Season 4.[42] Digital HD cameras were used for filming beginning with Season 8.[39]

https://en.wikipedia.org/wiki/Stargate_SG-1