Carnival treats the passenger as though the are already the higher status when they first go onto the ship if they are going to change to that higher level during the cruise.
IOW If the passenger will be changing to Platinum during the cruise all the Platinum goodies are gifted from the first day: free bags of laundry, the number depending on the length of the cruise, a gift, a free drink, the elite party, the special Elite line at Guest Services etc. For a cruise of 12+ days there are 5 bags of laundry.
This doesnt apply if the passenger will move up the last day of the cruise as it is not a full day.