It's definitely not bad to change your art style! In fact, that's how art styles are made. I'd much rather see myself improve comic-to-comic than stay in the same groove for the sake of unity, because that's just how I improve.
As you may know, cartoons are a little different than web comics. On a cartoon that you can watch on TV, what you're seeing is a group effort held together by mutual understanding and rules. This is why it is sometimes deemed unprofessional for cartoon shows to change their style. Web comics, however, don't have to have rules! You call all the shots and, to me, that makes them good shots.
Whenever I see a comic change styles, I'm always so happy for the artist! I love seeing someone improve/explore/experiment before my very eyes, and that makes me want to read your comic even more!