Is it possible to go from a full-time student to a part-time student?

Part-time vs. full-time status is determined by the number of credits you register for each semester. There isn't anything special you need to do to go from one status to another - taking 9 or more credits is considered full-time, taking fewer than 9 credits is considered part-time.

Changing your academic load may affect your financial aid so you should contact the appropriate financial aid office if you go from part-time to full-time (Loans: 212-998-4444; Scholarship: wagner.financialaid@nyu.edu)