Hi

I had seen on this notice on Prometric site for any rescheduling or cancellation of exams; you will be charged $70 fee

Effective 1 April 2012, at 12 noon (Eastern Daylight Time), a US$70 fee will be charged if you reschedule or cancel your exam within 30 calendar days of the appointment. 

So now it seems that those appearing for the pmp exams have to follow the test taking schedule unless there is very high emergency for rescheduling/cancellations within one month. 

Regards