After the 10th day of a fall or spring semester (census day), the Graduate School does not allow any course registration that will add credits to your schedule, except for "documented medical reasons or other verified, unforeseen grounds of personal or family hardship" (see REG 02.20.02). However, it is possible to swap one course for another after census day, as long as the total number of credits in your schedule does not increase.
I suggest that you make every effort to finalize your schedule by census day. The department will allow you to swap a course for another (or for research credits) only during the first two weeks following census day (i.e., by the end of the fourth week of classes). To do so:
The Graduate School has final approval of all Schedule Revision requests.