Personally, I don't think they're throwing away Drax being programmed to kill Thanos, I think he has just made a lot of grudges in the meantime. They really want to make Ronan the main villain in this movie
Just get H. Jon Benjamin already. He's a great voice actor, his voice fits the character, he's cheaper, has tons of fans that are in your demographic, and the average moviegoer won't care/won't know that Cooper's voicing "the raccoon".
There're some rumors that someone else's been seen playing/mo-capping Groot & that Vin Diesel might actually be running cover for him playing another character (especially since there's been such a big deal about Bradley Cooper playing Rocket Raccoon, and Groot is the only character that's received basically no public confirmation about his role).