I think I said this after issue #1 hit, but Steve McNiven has been confirmed to be off Guardians of the Galaxy by Marvel EIC Axel Alonso.
It's also learned that Guardians of the Galaxy #7 will be delayed, but will see another Marvel big gun come on board with Olivier Coipel for a "sequence" (i.e most likely a double-page splash).
Alonso offered the following at CBR:
Unfortunately, Steve will not be returning as he's hard at work on "Uncanny Avengers." Steve helped give us a great launch and, based on the feedback we've been hearing from readers, the transition to new superstar Sara Pichelli has been smooth.
And just to get all the cards on the table, since we also announced this week that "GotG" #7 is a bit delayed: Sara was hit pretty hard by a nasty flu bug over the past few weeks, and we've tried our best we can to hold the book for her, since she and Brian [Michael Bendis] work so closely together and he writes for her strengths. [Editor] Steve Wacker huddled with the folks in talent management -- C.B. Cebulski and Dan Edington -- to devise a creative solution that will actually see superstar Olivier Coipel come in for a powerful sequence involving Star-Lord and Thanos. But I've probably said too much.