This link pretty well covers the vast majority of cast changes, they are obviously bringing in more of the backstory and potential romance etc for walter. Most of I think cones from extreme lack of dislike that was out there for the saffron burrows character...

Oh yeah, she was really annoying. Don't know that I find a 'friends with benefits' relationship all that interesting, though... kind of kills the UST factor dead ;)

but hasnt fox always screwed with fringe?!

Oh hell yeah.