Is it cheaper to upgrade at the port?
It depends on the size of your group and the destination room, plus the popularity of the cruise. When you upgrade from home, what you are doing is paying a new individual fare for all passengers, for your new room, at the rate that was valid when you booked the cruise in the first place (since
DCL raises the cost of the cruise steadily as it fills up).
When you upgrade at the port, you pay an amount based only on the room you booked (or were assigned) and the destination room, regardless of the number of guests and the cost paid for the initial room. There’s a fixed price matrix they use to charge the upgrade cost.
Therefore, upgrading a 2-guests party at the port will usually result in minimum savings, even when making an important upgrade in category, unless you booked a discounted rate (FL, military or *GT). You should modify your booking from home to an available stateroom that you would prefer instead of taking a gamble at the port.
If you are upgrading a 4 or 5-guests party, it will result in much better savings. If an interesting stateroom is available at the time they stop selling them online (3 days out I believe), you should try to arrive as early as possible at the port to try and get it. Of course, under the current rules, you can’t do that and are tied to your PAT.
I’m always sceptical of reports that discuss the quality of the discounts offered at port because there are many variables involved and not many occurrences. The only valid comparison would be the same upgrade, for the same group composition, for a similarly priced cruise, booked at the same time in relation to the cruise date.