This shows how much I was into COW AND CHICKEN when it was on, because as a kid, it was the first time I ever noticed off-model animation, I even posted about this on TV Tropes, but there was certain episodes where the animation and character designs differ noticably from other episodes - in fact, the first episode ("Field Trip to Folsom Prison / The Girls Bathroom") is one of them but because it was the first, I suppose that can be forgiven; but other episodes, like "Sumo Cow" and "Yard Sale" almost don't even really look like Dave Feiss's style.
I will say this though, just about all CN shows were really consistent with their styles and animation, though some obviously went through some natural evolution over the years, and C&C was one of them: notice how pastely and "clean" the first season looks compared to, say, Seasons Three and Four, where the colors are darker and bolder, the lining is stronger, and the backgrounds have a textured look to them. EEE is another example, which Danny Antonucci even pointed out; Ed is perhaps the most noticable, if you look at how he used to have smaller eyes, a thicker eyebrow, and a more square-shaped head.