Kind of. If you become Premium, you are not guaranteed those exact prizes right away, but those prizes are in the prize pool, so there is a high chance of eventually getting them. The sooner you become Premium, the higher the chance you have of getting every item in the Premium Prize Pool, as there are no repeat prizes and there is only enough prizes in the pool for each day of the event. So if you are Premium from the beginning of the event, you are guaranteed every Premium prize as long as you donate at least once each day.