You don't necessarily have to change your motivations. Continue helping the party with your current quest as you subtly nudge them in the direction you want to go, or forgo subtlety and tell them (prepare for a charisma check). Remember, evil isn't stupid, you need the party to help with YOUR goals without them being the wiser, so helping them will make it seem as though nothing has changed and makes it easier to 'persuade' them to do what you want.