It's just Stark who builds Ultron instead of Pym, similar to what they did in Next Avengers. That's all, if they're going to do it. Just leave Ant-Man out of it entirely.
No, just no. The fact is, Ant-Man is getting made and it is going to be set in the MCU so he is going to be the creator of Ultron. Since Pym is almost exclusively defined by his membership in the Avengers and his relationship to Ultron. Without those two things, there is no reason to make the film.
BTW, Next Avengers sucked.