I thought the art started changing a long time ago - more like around Loguetown and then again some time during the end of the Arabasta arc. I liked how most of the characters were eventually drawn differently from their original appearances (especially Ussop and Chopper).

Interesting observation about Luffy, Nami, and Sanji's faces... I never really noticed that they were similar. It's probably because I tend to focus on the eyes which are very different between the three.

However, there are some times where I really don't appreciate the art in the anime. Most of the time when they are adding extra scenes or doing filler episodes the characters seem to be drawn from memory or without a reference and they end up looking very odd.