Why does Evil Buu change when he eats Fat Buu?

I think your first option could still work Tienshinhan. Remember that when Goku and Vegeta were inside of Buu, it was confirmed by the mini-Buu they confronted that Good Buu was given special treatment. He was sealed away in a pod, and that is how Evil Buu was able to transform into Super Buu. Dabura was just eaten, and not put in a pod, but killed, like Chi-Chi, Videl, Bulma and all the other humans Buu devoured were also killed...

I think Fat Buu actually is pretty dumb though. It happened more than once that Babidi had to act lika a tolk, or translater for him when somebody addressed him. But even though he does not appear to be very smart, that doesn't mean he's not a gifted fighter, as he obviously is. Instantly learning the Kamehameha by just seeing it proves that much. Fat man, in that respect, makes me think of Goku a lot. Goku also learned the Kamehameha by just seeing it in action, and he too was lacking in the brain department, but had great fighting instincts all the same, much like fatty...