Yes, I've heard of upgrades being available to anyone at the port - the reason being it's a whole different system. Even after booking with points or with a TA, you are the one in control at the port - they don't have access to those reservation details, as I understand it.
The restricted GTYs are a new thing, and while it's true that you can't modify
anything before that, once you get to the port it's a level playing field.
Port upgrades are not always available (none were on the ebta or the first Med cruise), but just in case there are some, you want to arrive EARLY and be one of the first in line!! There is no predictor for rates & availability, as far as I've been able to find, but it's always worth a try.
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