I think the cuteness started to die in the third season with Dewey's Dog, but there were some definite scenes in the fourth season that showed he hadn't quite completed the transition. For example, his talk with the Sunday School teacher in Day Care still showed some traces of cuteness, especially with the "giant shovel." And even in the fifth season, such as in Thanksgiving, it's clear that he can at least pull off the cute act as a scam.
No matter where the transition occured, I'm glad it happened. I loved the cute scenes, but those started to lose their appeal after a while. And, without the change, we never could have experienced the humor of Dewey's Opera or Ida's Dance.