questing about "authorized stay" during pending period? Thanks f

I am considering to change my current status to a new category. After reading PJiang's post, I still have the question: am I in the "authorized stay" status if I file the application before the current expiration date on my I-94, and it is pending in USCIS? Or I have to get the Receipt Notice Letter before my current expiration date so as to qualify for the "authorized stay" status? Thanks a lot!