I guess they changed it to freshen up the look of the boxes but I wish they used something different than that. Either way, I'm a "It's just a box" type of guy.
While I would rather have some continuity, there is still some. Look at the bottom of the front cover, it is implied that there is graffiti. When the 7th season started, Comedy Central started having graffiti infomericals and junk to incorporate it with South Park. It's not a definate explanation, but it's something.
Well, judging by the "Complete 7th Season" font at the bottom of the cover, I have a feeling that the spine will look similar to the 6 preceding seasons. So I don't think uniformity will be an issue here. I do agree though that the front cover looks hideous, but if the spine remains the same, my complaints will be minimal.
It seems likely that when you remove the grafitti slipcase, it'll have the kids standing there like the other ones. There's almost some continuity that way.