No, you will only receive retired NC items from the windows you open while being a Premium Member. If your Premium Membership activates after you've opened some windows, you'll need to use another ticket to open the same window to get your Premium retired NC item. Make sure you're Premium before the start of the event to get the most out of each window!
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!