You can take an underload in your last (graduating) semester. This means registering for less than required number of credits and just enough needed for graduation. You will still be considered full time. However, this requires prior approval. See the graduate handbook. However, you should really make sure that you indeed graduate in that semester. There can be serious immigration related implications if you do not for some reason (e.g., you fail a course). If you are not confident, it is best to register the required 9 credits.