There is no requirement for differences training between SEP types (except, of course, for tailwheel, VP prop SLPC, etc.). If, having undertaken the skill test on a C150, you wish to fly a PA28 then you need to undertake familiarisation training, which is undefined and amounts to generally no more than reading the POH. There is no requirement for certification of familiarisation training, nor for it to be supervised or given by an instructor.