I use the BindPad addon, so the length of this macro might be too long, but this is what I use on my Paladin to swap level 90 talents. This particular one unlearns the first or second level 90 ability and learns the 3rd. You'll need to change the talent Row and Talent numbers to suit your needs.
You'll need to have opened the Talent window at least once for this to work. Obviously also don't work in combat.
/click PlayerTalentFrameTalentsTalentRow6Talent1 RightButton
/click PlayerTalentFrameTalentsTalentRow6Talent2 RightButton