Um, I think he can if he is speaking excathedra or whatever the right word is. I do know the Pope can make changes when speaking on matters of faith and morals from his position as Peter's successor as leader of the church.
No
And he certainly cannot change teaching on homosexuality and not on a plane in the middle of the night talking to USA Today or whatever paper anticatholic freepers will take as bible truth in this case
It would help if we could just stick with the subject her