User blog comment:JokerJay779/Is Sega redesigning the Sonic characters?/@comment-6671743-20130221042113

The artworks are just to represent occassions of a each month. Amy's is February and Valentine so she'll be wearing winter clothes with her Piko in a valentiney theme. Sonic's is January so that's a month mainly for snowboarding and snow. Cream's is representing Chinese New Year (Or Spring, yeah I think it's spring) As for the art-styles, they change slightly every year.

http://www.planete-sonic.com/IMG/jpg/a305_wp119_1920x1200.jpg - 2011

http://www.planete-sonic.com/IMG/jpg/a305_wp71_1024x768.jpg - 2008

I personally think the older style looked more solid.