I don't think it's easy for them to "move on" from something they've invested so much time in. I heard the ending was badly written, contradicts the canon of the series, pulls at LEAST 2 deus ex machinas out of its ass, and all the the player choices that were supposed to be building up to that point didn't really seem all that relevant. I heard there were slight, minimal changes, but it wasn't exactly what Bioware promised. They didn't lie about it, but they really overhyped the whole choice aspect of these games, and to have it ultimately meaningless...
Well, I can say, without actually playing the games, I feel their frustration.
In regards of actually changing the ending, I think it's really up to Bioware. If they were satisfied with the ending, they might be right within their mind to change it to the way they wanted it to. Perhaps not the absolute BEST ending that EVERYONE wants, but at least an ending that's not quite as crappy. Not to mention, if they stand firm on their ending, they'll probably end up losing costumers in the long-run. Mass Effect is one of their BIGGEST recent franchises, and people are going to remember the ending more than anything else in the game, and will likely continue to hold a grudge against them for it. That's not to say EVERY fan overreaction should be listened to and dealt with, but it's still worth getting some fan input on what does and doesn't work... from people who know what they're talking about.
