SG-1 has only been HD for either the last 4 seasons I believe. I think season 7 was the first to be shot in HD but I'm not totally sure.
All of Atlantis has been HD.
I guess earlier seasons were shot on film. I guess that means they would have been transfered to video via HD making all seasons HD. So I guess I don't know what I'm talking about.
I believe you were closer to being right before you edited this
I know they started off with 16mm film. At some point (3rd season?), I'm pretty sure they went to some flavor of 35mm film. Near the end (perhaps 7, perhaps later, I do not have the info onhand) they did start shooting with HD cameras. As has been discussed in other places, anything done on film could be re-scanned into high-def. However, I seriously doubt at this point that SG-1 has had this done -- that would have been an awful lot of expense for a TV show. Someone could certainly go back now and re-scan SG-1 into HD (like they did with Star Trek: The Original Series), although I would imagine all the effects shots were done in SD video and would only be able to be upconverted instead of being "true" HD.
The episodes of Atlantis that have shown on UHD were filmed in HD, but I'm not so sure about the effects shots. I suspect those were also done at a SD resolution, being that at the time the only way to view the show was in SD. It always seems to go very soft during any kind of special effect (look at the gate in a regular shot when no effects are going on, and then look at it during a "kawoosh" - all the symbols around it go fuzzy)