Service center usually allow to have walk in finger prints if that center is not crowded on that day. You must have the finger print notice with you. They may/may_not ask you the reason behind early finger printing. If they ask you tell them the truth. They usually do not care as long as they are not busy and you are already scheduled for the printings in the future.  You may try your luck couple of times if you fail to have finger printed at the first time.