C172 -> PA28-161 Time needed to convert?..
My sign off from c152 to pa28 was a couple of flights with an instructor - one in the circuit with different approaches (normal/flapless/glide) and one doing stalls just to note the different characteristics in the stall. I think you could probably do it in one 90min flight (pilot dependent) if you have read the POH beforehand.
Main differences I recall are switching fuel tanks every half hour and adding fuel pump into your pre take off and pre landing checks. Also a bit more float in the flare due to lower wing and more pronounced ground effect.
Overall I prefer it to 152/172.