Dreaming of a White Porsche Christmas - S10-E6
Corrected entry: Because of a Christmas wish Stan becomes Principal Lewis. While running away from the CIA Avery Bullock yells out "Smith, stop or we'll shoot!" Stan hadn't changed back to himself yet. (00:14:40)
Correction: Stan doesn't become Principal Lewis, they just change lives (e.g., decisions they made, careers, lifestyle, etc). This is evident by the fact that his name is still Stan and Brian Lewis is still Brian.