Wasn't annindoufu only illustrates imas merchandise, but never any part of the game?
It's still his work on master artist 2 jackets so it's unlikely that he's been replaced.
If it's true though then it's a shame. I have that brilliant idol artbook of his & i really like his art.
I'm not entirely sure on everything he did, I just know him as the illustrator for iM@S (at least up to this point). I know that, in his artbook, there are the designs for the SP DLC (prior to the fan-costume contests), so I had assumed he had done the in-game content for iM@S360 as well.
I don't know if the company ever really says anything about replacing their illustrator. And I know for sure that Mr. Tamiya Kiyotaka emulates Annindoufu's style, because it's the "iM@S" style.
It's really noticeable for the 2010 birthday banners and that Kotori sketch from the anime page that there's a different illustrator. To me, the art on the MA2 covers looks similar, but different than Annindoufu's style (the closest one, to me, is Mami's cover). Nonetheless, whether it's obviously Tamiya Kiyotaka drawing or not, it's still got that same iM@S style to it (the eyes and mouth, noticeably)
Artists' styles change over time, though as a professional illustrator, the ability to consistently draw the same is key, which is why I think that Annindoufu isn't the main illustrator anymore.