IF the terminal is full, they can and will require that you wait until your PAT or the terminal uncrowds a little with people boarding. If the terminal is not full, they will let you in.
BUT.
Your Boarding Group is tied to your PAT and generally they will not change the Boarding Group. Two years ago I was checking in at PC with the earliest PAT having come on the first bus from MCO. There was a family at the counter next to me who had been on the same bus but whose PAT was much later - like 12:30 or so. I was given Boarding Group 3 as there were a lot of Platinum and Gold cruisers on that cruise. That family was given something like Boarding Group 43 - as soon as the agent gave them their card and started to explain the process, the father started waving around the card and loudly fussing that they were going to have to wait so long after making the effort to get there early and eventually a supervisor came over and politely explained to him that the Boarding Groups were assigned based on the selected Port Arrival Time, and unfortunately because they had selected a later time, their Boarding Group was higher. So they ended up standing there watching people who arrived after them board the ship.
Now, had the father been polite in his first questioning of the boarding group my guess is the agent *could* have given him a different card. But because he immediately started yelling, that very likely eliminated any chance of that happening. But I don't know how much leeway they have to change the Boarding Group number, so if you are arriving far ahead of your selected PAT I would be prepared to have a higher Boarding Group number and spend a good bit of time in the terminal.
