So you really need an instructor to change from a PA28 to a C150? i think not. These are PPLs not students!
Pace, how many PPL conversions/checkouts have you done? I can now jump into just about any single and fly it OK without reading the POH (if there is one I DO read it, but have flown aircraft that do not actually have one), but remember being very concerned converting just after qualifying and think that is the norm for most!