Having converted many pilots from the Cessna to the PA28 there is really only one significant difference, the PA28 series has a stabilator which remains effective to low speeds so you do not need as much aft elevator on landing. The slab wing PA28s also pitch down when you apply flap, wheras the taper wing varients pitch up. A bit of general handling and a few landings is all thats required. As you've flown the DA20 as well, you should find the transition straightforward. Reading the quote above implies that the PA28 is actually a very safe aeroplane!