No, you will only receive retired NC items from the gifts you open while being a Premium Member. If your Premium Membership activates after you've opened your gift, you'll need to exchange another boarding pass for the same gift to receive your Premium retired NC item. Make sure you're Premium before the start of the event to get the most out of each gift!
Please note that sometimes Premium takes a few moments to "kick in", so it may not apply to your account right away. Please make sure Premium is active before purchasing any prizes, as if it isn't, the bonuses won't apply and you may need to purchase them again!