Payback does not have any priority. If the opponent switches then I believe Payback will hit the Pokemon that switches in*, rather than hitting the Pokemon on the way out (like Pursuit does). Therefore its priority never changes.
In the case of Baton Pass: if the Baton-Passer is faster they will use it and switch out, then Payback will hit the next Pokemon. Prior to Black/White it would also hit with double power. If the Paybacker is faster, then they will hit first and with regular power.
* If anyone can confirm this it would be greatly appreciated