Why was he replaced? Most of the other voice cast have remained the same.
This was explained by Brad Bird (the director of the movie) at the D23 Expo in June 2017:
Director Brad Bird revealed at D23 Expo that, with the exception of Dash's Spencer Fox, the main cast from The Incredibles will return for Incredibles 2.
Bird explained at the D23 Expo animation panel that they needed to recast Dash in order to make sure his character still sounded like a 10-year-old boy. Incredibles 2 takes place a minute after the end of the original, so Bird wanted to avoid Dash suddenly having a much deeper voice.
Spencer was ten or eleven when the first movie was made, and now he is twenty-five: thus he doesn't sound like an ten year old anymore (which Dash is supposed to be).