You COULD, but it would be really unwise. You need to ensure that there is a very specific and even amount of fork oil in each side, and by pulling out the previous spring and installing a new one, you would most certainly be changing the fork oil level. Your best bet is to drain the oil, replace the springs, and put it back together again ala the service manual.
There'd be no point in purchasing the pro series suspension kit if you're not going to do it right.