Just monitor the availability of the seats in May from the website day by day. Some seats may be released. I ever did that before and it worked. From this website: http://www.mba.com/mba/TaketheGMAT , you can check the seat availability, and reschedule the test date.
You can reschedule the date in the www.mba.com. It will only take $50 if you reschedule the date one week before your original test date. If your planned date is not available, just wait for couple of days and it might be available later. So don't worry about that.
