Well, for one thing, if you upgrade from a JS
to a GS, you will get access to the Concierge Lounge, if the ship you are on has one. They have these lounges on all Radiance, Voyager, and Freedom class ships. If you enjoy a cocktail in the evening, they would be complimentary from 5:00 to 8:00 PM.
I wouldn't count on being able to upgrade at the port. You're better off to use your TA. Upgrades at the ship are few and far between.