Why did they change the reason Eddard goes to kings landing?

[Padraig]

Cat says the 'you can always say no' line, but the discussion goes no further than that.

Apparently that line is clarified so that its clear that she wants Ned to say no (admittedly, I haven't seen the clip).

It'll be interesting to see how her other lines in E1 are changed on foot of that. She might still acknowledge the advantages of a Sansa-Joffrey marriage, which will make people happy.

[OhDanyBoy]

Further to what I said above, I expect the scenes shown where Cat is crying, and says the lines about honor, choosing, last time you came home with Jon, etc... are from the second episode, after Bran's fall and before the departure for KL. This would make sense, as nothing like this is in the leaked script, and would fit her 'falling apart' after Bran's incident, and the 15 minute screener was taken from the first 3 episodes.

Oh, and it seems this entire hullabaloo is centred around basically a single line from Catelyn, in what is a 3-page (3-minute) conversation. I'd be willing to bet dollars to donuts that this is an overreaction, and that the show will still display Cat's initial support for accepting the Hand position, only to change her mind when Bran falls.

[Lord Varys]

Well, I reread the leaked script of the pilot this morning, and although Cat says 'you can always say no', she is there the driving force (together with Luwin) to convince Ned to go to KL when the letter arrives. But the important thing for her is that 'he would make our daughter Queen'. If this is still in the pilot, I'm happy again. Then I'd read Cat's original 'You can always say no' line just as the comforting words of a wife who knows her husband and his feelings very well.

[Horus Ex Machina]

The change to Ned's character is minor, IMO. He could still be reluctant and in need of persuasion, only now it seems like the task of persuasion falls to Luwin alone instead of Luwin and Cat (this is my guess since I don't think we've had a clip of Luwin actually talking). He still seemed hesitant when Robert first makes his offer in person, and possibly ruefully cognizant of Robert's intractability when he's talking to Catelyn in the godswood.

As for the change to Cat's character, Catelyn is not the most important character in that obvious way Jon and Dany are, more than a few people say her main purpose is to report on Robb. Nor does she have the greatest plot arc like Tyrion or perhaps Jaime, the mechanics are pretty ad-hoc if not incoherent. She doesn't often turn up in collections of people's favorite lines or most memorable scenes, barring the obvious. Her character brings the human element to the table, she's the real human being of the story and the realization of her nuances, the craft of her character and consciousness, was her greatest contribution. I must say I don't see why they would make it a priority to supply later on what was lost in these early scenes if it wasn't a priority early on in the first place, and I personally am kind of thinking that later on her sense of duty and honor and her stiff upper lip will come off ... smaller and lesser in stature, somehow, but that's baseless guessing on my part.

I saw her as the dichotomy between the thinking of a Northern lord vs her Southern, Andal, upbringing. It wasn't so much Catelyn's ambition that sought out a marriage for Sansa to tie her to the crown so much as her perception of familial duty. To them, the best way to secure the safety and longevity of your line is to tie it to the strongest match possible. Her duty to Sansa was to ensure such a match. Of everyone in the Stark family and household, she is as close to an outsider as you can get. This is highlighted to the best degree in the scene which takes place in the Godswood. So while it seemed natural for a man like Ned Stark to isolate himself from the Southern courts, a woman like Cat who was raised there wouldn't see it that way.

[Lady Blackfish]

It's not this burning, avaricious ambition, no, but it's different from Ned's belief that the further away from power, the better. As you say, the reason it's familial duty to marry your children well where Cat comes from is that there's an advantage to maintaining a certain amount of power, so it's embedded in the practice, I think. Longevity of your line is technically possible without status, after all (though it must help). Still I agree with this:

I saw her as the dichotomy between the thinking of a Northern lord vs her Southern, Andal, upbringing.

I really don't think she should come off like a northerner, she isn't one.