I withdrew from my course; can I now change that W to a Pass/Fail grade?

No, if you have withdrawn from the course, you will not be eligible to request the Pass/Fail grade for the course. Therefore, it is very important that you talk with your instructor and your advisor to choose the best option for your situation, whether that be to stay in the course with the grade, request the Pass/Fail grade or an incomplete, or withdraw from the course.