If you want the flexibility to change your mind about the insurance products you select from Extend Health after your enrollment call then DO NOT ENROLL BEFORE OCTOBER 15,2013. It is a quirky Medicare thing. If you do you will be using a one time Medicare "Special Enrollment Period" aka an SEP that is provided because IBM's group insurance is going away. You are not allowed to make changes after that one time.
If you enroll on or after October 15, 2013 you will be enrolling during Medicare Open Enrollment which runs through December 7, 2013. Then you can make as many changes as you want.