Using a spring compressor is the safe way to do it. If you have the OEM springs on the car right now, you could always just undo the center nut on the top of the shocks (wear eye protection, cuz the top nut may go flying). The extra weight of the car on the spring will keep it from popping quite as violently as it would if it were off the car. Then remove them from the car and install the new springs. You shouldn't need a spring compressor for the lowering springs because they are shorter than the OEM ones. However, if you need to reinstall the OEM springs, you will need a spring compressor. If you haven't had a chance, check out my "How to" I just posted on replacing the rear shocks and springs.