May I change to a part-time jo

Hi, every one, I have a question which needs your input. My 140 was approved two months ago, and my 485 is pending since last November. I don't know how long it will take to get approved. Now I am a part-time phd student and my university requires me to be a full time student for at least one year to do the teaching assistant. So I want to be a full time student from next fall because I want to granduate earlier. I am sure my current employer will provide me help for a "future employment" letter. My question is: can I change to a student who only works 20 hours a week? If the RFE comes out, my current employer's "future employment" letter will be fine? I appreciate your information.