from the srd:
Code of Conduct: A paladin must be of lawful good alignment and loses all special class abilities if she ever willingly commits an act of evil. Additionally, a paladin's code requires that she respect legitimate authority, act with honor (not lying, not cheating, not using poison, etc.), help those who need help (provided they do not use the help for evil or chaotic ends), and punish those that harm or threaten innocents.
Associates: While she may adventure with characters of any good or neutral alignment, a paladin will never knowingly associate with evil characters. A paladin will not continue an association with someone who consistently offends her moral code. A paladin may only hire henchmen or accept followers who are lawful good.
Ex-Paladins: A paladin who ceases to be lawful good, who willfully commits an evil act, or who grossly violates the code of conduct loses all special abilities and spells, including the service of the paladin's warhorse. She also may not progress in levels as a paladin. She regains her abilities if she atones for her violations, as appropriate.
So
A paladin who chooss to move on but does not grossly violate the code, stays lawful good, and does not commit any evil acts should still retain their full paladin powers even if they choose to become another character class, a politician, a lawyer, etc.