It's called a retcon, and it's not exclusive to Stargate, although it reared its ugly head in this particular franchise more often than usual.
Let's look at some more SG retcons, shall we?
1. Initially, according to "The Enemy Within," the Goa'uld were able to shed most of their bodies like a husk and actually fuse what little was left with the host's brain. This was not contradicted until the second episode of season two, "In the Line of Duty."
2. And what about the sarcophagus? Remember when the movie and season 1 strongly implied that using the machine resulted in addiction and that it slowly drained your essence, and that's part of what pushed the Goa'uld over the edge? Yeah, me neither.
This retcon is a clear example of the writers wanting to shed the convienience of a device for the sake of drama and to keep the sarcophagus out of the SGC for good. No easy healing here, folks!
3. The Asgard being greys. Seriously. I grew to accept this, but my hopes of seeing vikings in space ships were dashed by season 2's "Thor's Chariot."
4. Remember when the Oannes were instrumental in Earth's rebellion? And then "Moebius" happened. . .
5. Crystal Skull.
6. The writers never being able to decide which ancient culture should be used to best represent the ancients, thus coming up with "OH THEY INSPIRED ALL ANCIENT SOCIETIES K?" Way to ruin "Maternal Instinct."
7. I like how the Replicators were first an originless enemy of the Asgard, then we found out Reese created them as toys, they destroyed her planet, then they left the galaxy, THEN we found out that Reese created them because her creator was maybe an ancient and the ancients created the replicators to fight the Wraith. How fucking convoluted.
Of course, there are many more. Yeah, this hasn't been a consistent franchise.
All true, but if you think that is bad, try the retcon in Star Wars. Ignoring the issues with other LucasArts productions such as the video games, in each release of the original trilogy, Lucas has retconned something else. Examples:
1) Han shot first.
2) Han says, "Imperial Cruisers, our passengers must be hotter than I thought."
3) Medichlorians? When did Obi-Wan say anything about micro-bugs.
4) Old Vader morphed into Young Anakin.
There are more, those are just the ones off the top of my head, and some of the bigger ones. It gets really fun when you add in expanded universe, which has almost completely retconned the movies out of existence. It's the price of shortsightedness. An idea gets made, but in order to not have loose ends, they wrap it up nice and neat, then if its a success and they make a sequel, they have to make changes.
My issues with SGU are that they aren't even true to their own retcons. Most of the retcons from the shows are the result of TPTB now, and they ignore all of them. Then they retcon new things, based off of other retcons, that are in conflict with themselves. Examples:
1) Lucian Alliance. How did they get to Destiny. Icarus was the only place that could do it, and it blew up. Oops.
2) Ship flies through the sun, but oops, a passing paint chip from the NCC-1701 Enterprise strikes the ship and it falls apart like it were built out of legos? Whoops.
3) Ship exists for millions of years, alone is space, yet all the secondary systems still work, but oh yeah, all the primary ones have failed? Huh?
4) Crew goes through the gate, only to find that Toyota has beaten them there, and the aliens are driving around in indestructible Tacoma trucks. (I don't know what car was filmed on the alien planet, but Toyota trucks are a running joke from Top Gear, so it works)
