Mordekaiser's Q is totally different, his W acts completely differently, he can now have the dragon as a pet, and he goes in a completely different role than he used too.
Darius got his passive completely changed, his Q got changed, his W got changed, his E got a QoL change, and his ultimate got changed. I don't want to go into literally all of the changes, but all of them are significant enough to mention.
Skarner got a totally different passive, a lot of stat changes, his E got his old passive, I don't remember anything else because I never bothered to read up on it.
Garen didn't get changed that much, except for a lot of numbers shifting around and getting an entirely new mechanic on his ultimate.
Garen MIGHT be considered to have gotten small changes, besides that, no, they were all quite significant.
And how is Mordekaiser not as updated as Tristana? Did she not keep pretty much everything except the one change in her kit that gave her a bomb?
It's unfair to bring up Sion, he got a complete relaunch and was changed more than any other champion ever has been in the game.