Yes you can as they will hold your item for 7 daysI have in-store pickup but my status still is on "Order Placed" with the message "We’ve received your payment information and are preparing your order.".
Also for some reason my case has a different pick-up day (20th) than the phone (22nd). I assume I can just get them on the same day? The Apple store is 40 mins away -- I'm not making an extra trip for a case...