@xanni15: This is really not fair to the fans or the character, this is like making a film about Superman getting a side kick then the Joker beating him to death with a crowbar then said sidekick coming back as an enemy. If you cant introduce Ultron right why use him at all? Why not use Kang or someone else? They can't even mange to slip Hank Pym in to the movie yet they can fit in fake Quicksilver and fake Scarlet Witch? What a joke.
I don't think it's that big of a change, but I respect your opinion. One character's origin story is being changed to better fit in with the rest of the Cinematic Universe, that's what it boils down to. We have no idea how Pym's solo movie will turn out, if it bombs then Marvel has an extra character around with Ultron being attached to Stark, if it is successful then Pym becomes a main part of the films going forward and gets to flesh out more of his character. Slipping in Pym is no small feat, especially if you want him to be Ultron's creator. Can't just say "Here's Dr. Pym, he created Ultron, now watch as he disappears the rest of the movie." Also, we still don't know all the facts associated with Ultron's creation in the movie, perhaps Whedon is holding major stuff back. Based off Avengers I can't help but have faith in him.
I don't think it's that big a a deal, even for comic readers. We all have long ago accepted that our favorite heroes and villains have to be adapted to be put on the big screen, and that sometimes for the sake of the overall story one of two might be slighted. Marvel has shown an incredibly ability for long term planning in their Cinematic Universe, whether its making their characters not as powerful as their comic versions, connecting all of the movies and characters via cameos, focusing on one over the other, or making changes to characters (the Mandarin, moving on to a new Hulk, Ultron, Electro, Nick Fury, etc.). I personally don't see the reason for outrage, Marvel is making what they deem is the best choice for all fans of their movies. Tony Stark is far and away their most popular character right now, thanks to his movies and general exposure, so continuing to focus on him and allowing him to create Ultron makes perfect sense. Marvel's building characters, showing back-stories, and allowing them to make mistakes and grow from them. Stark's progression throughout the movies has been great to watch. Too much is at stake to slip in Pym's character into Avengers 2, and seeing any movie (not just those related to comics) make changes to characters and the story isn't anything new in life. The fact remains that comic readers are not the only people who watch comic book related movies.
Pym will have his shot with his solo film, if he delivers that will force Marvel to make room for his character among the "big guns" in the Cinematic Universe. Every character has a part to play, none off them are bigger than the team (something the The Avengers showed us) Whedon created in my opinion one of the best comic book movies ever with The Avengers, and word is he wants the sequel to be more character driven and focused on the characters. I have absolute faith in him to bring about another amazing film, one that surpasses and improves upon the previous. Whether it is Pym or Stark creating Ultron, I can't wait for the movie to be released.