You must get at least -100000 reputations points with your own nation
You must visit a governor
The governor gives you a warning
From now on three things can happen when you visit a governor:
You have lower reputation points then at point (1) -> You will be excluded from the nation and become a Pirate.
You have more reputation points then at point (1) -> You will not be excluded.
You have more than -100000 reputation points -> No more danger of a nation exclusion.
If you belong to the Portugal nation, every time you attack a player (you shoot first, making him 'Defender') you will lose a large amount of reputation with Portugal. Once you hit negative reputation with Portugal sinking any nation PC or NPC will result in your turning Pirate.
Date of your last nation change needs to be at least 60 days ago (or the governor will deny your request).
You need at least 500000 reputation points with the destination nation.
You must visit a governor of the destination nation
The governor accepts your request, you join the destination nation. You get -250000 reputation points with your old nation (but only as long as your reputation points are positive).