>>12389045Hate to ruin your fun, OP
But they're all drawn by Sugimori.
Art is a constantly changing shaping and reshaping shambling beast or horror. As an artist I can tell you right now that it's just simple progression and influence. His art back in the 90s was heavily inspired by Akira Toryama (his favorite artist at the time, apparently) and his art now has taken influence from some of the other popular Japanese artists in the media.
Just because it's different looking, doesn't mean it's a different artist. It's completely natural. You'd understand better if you were an artist, too. Once your art starts stagnating you either change it or you fall into mediocrity. It was clear by gen 3 that his art style was stagnating, so he changed it. That's all there is to it.
He does have other artists do character and pokemon designs for him, though. But he still has final say and is still where all official art comes from. 
