^^^ OP, congrats on your upgrade! Sounds as though you had a fantastic time on your cruise. Kudos to you.
Are upgrades only possible for those who arrive at port early?
Its not a given that any upgrade will be available no matter when you arrive.
Let me give this example.
Every room is sold out, the ship is full however a person in a category 1 room is sick and cancels the day before the cruise. Bingo, one room is now open.
Lets say someone in a category 3 room inquires about the upgrade and takes it. Now the category 1 room is booked however
..a category 3 room becomes available.
Next a person in a category 4 room inquires about the upgrade and takes it. Now the category 3 room is booked however
..a category 4 room becomes available.
.and so on.
Of course a person in a category 12 room can take the category 1 room so only one family could upgrade. However its my guess that this is possible but unlikely.
The person who booked a category 12 room likely has a tighter budget and they would need to pay a larger amount to upgrade to a 1 then a person who booked a 4.
Again, it could happen, I just dont think its as likely to happen.
So even if at one moment the category that you are seeking is not available, it may become available.
As far as the best time to get a upgrade, if you are seeking a top category I feel the earlier you get to the port, the faster you check in the better your odds but its no guarantee no matter what you do.
The only way to guarantee you travel in a given category is to book it up front. After that its a matter of luck and timing.
I only found out about upgrades after I joined Disboards. I upgraded this one time. I read many posts here at Disboards regarding upgrading.
One member here, screen name kcashner, has posted about her multiple experiences upgrading. If you would like to learn more about port upgrading Id recommend doing a search for her posts and reading the ones related to port upgrades. I found her information very helpful.