Nowhere to Run - August 10, 1968 - S5-E4
Corrected entry: For four seasons, we were told that Al always saw Sam as the person he'd leaped into. (Case in point: Al once suffered an attack of guilt-ridden lust when Sam leaped into a very attractive woman.) Here, that series canon is violated when Al tells Sam, "Nobody sees you except me. Everyone else sees the real Miller." (00:21:10)
Correction: There was at least one prior example of Al seeing Sam as Sam, in the episode "Miss Deep South". So the changed premise happened much earlier than written here.