Then ask he to release you from your oath (which he will do for a relation debuff) after that you will lose your holdings but you will be free from any responsibility to the king and your faction will be gone. If you ask a king to let you be his vassal then you will join his kingdom. Other than that you can only leave the faction and lose your fiefs. The only way a player can defect is by choosing the I have been wronged option.
