If you have already voted before becoming Premium, you can still get Premium Prizes for whatever Categories you voted for (as long as their winners haven't been announced yet) by returning to the Neopies and looking for the new button near the top of the page that says "Collect Premium Prizes"! This will reward you with the correct amount of Premium Prizes per open Categories that you have already Voted for. This will only be a one-time button, afterwards, you can vote like normal and will receive all your prizes right away.
IMPORTANT NOTE: You must press this Button before the winner of the Category is announced or you will miss out on whatever premium prize you received for that vote!