Hi Trustongod,
There are two 'factors' in rescheduling your GMAT:
1) You have to log-in and reschedule it at least 7 full days before the current date of your Exam. To be safe, you should log-in by October 27th to make the Official change.
2) The current fee to reschedule your Official Test Date is $50.
Since this is the 'busy' time of year, many Test Centers fill up their available appointments quickly, so if you have a specific Day/Time that you want to reschedule your GMAT for, then you'll have a better chance of booking that specific appointment if you try to do so sooner rather than later.
GMAT assassins aren't born, they're made,
Rich