For everyone mentioning Grand Companies and how they explicitly stated it as being a "one time choice," it doesn't really matter. Everything else was a "one time choice" when you made it too, whether the game told you so or not.
We aren't getting new grand companies though. A lot of people would have picked X race of Y gender if it was available. All the companies are there from the start so you know what you are getting. If we DO get new companies then yes we should be able to pick a new one.