According to Koji, when he was making the gba games a study was conducted that showed the artwork wasn't in accordance to the handheld they were on(being that the target gameboy audience are minors). Thus, the decision was made for the DS games to have a more kid-friendly look, since the age demographic that owns DS are mainly youngin's. On the other hand, he was asked if the decision to change artists was a good one; he stated that he believes him & his team half-succeeded, whatever that means.
Honestly, If the next DS game has similar artwork, will that stop any of you complaining from buying it? It's really not a big deal.