If you Vote for Categories and then become Premium you will receive the amount of Premium Prizes for all Caegories that have yet to have their Winners announced, depending on how many you voted for before becoming Premium. So while you will not receive any Premium Prizes for Categories you voted for if their winners have been announced already, you could potentially get between 1-3 Premium Prizes the day you sign up for Premium depending on how many Categories are still open and how many you voted for before signing up for Premium.
Example: You have Voted for all Categories 1-6, and on the day that Category 6 becomes open for voting you sign up for Premium. Because there are three open Categories (meaning there are currently three Categories whose winners have yet to be announced) you would get 3 Premium Prizes. So on this day whether you voted for Categories 4-6 or just Category 6 (due to having voted for the others already the days before), and then become Premium you would receive 3 Premium Prizes. But would not receive any Premium Prizes for Categories 1-3 as those Categories would have already had their winners announced by that day.