Obviously, the IBM enrollment period is over but I wanted to remind you that you do have a "window" provided by Medicare law (it has nothing to do with IBM) called the Medicare Advantage Disenrollment period and it is January 1- Feburary 14, 2012. During that period you can switch from IBM's Medicare Advantage plan to original Medicare. So, if, for example, you decide you hate Aetna's HMO or PPO you can switch to original Medicare during that period. You won't be able to get IBM supplemental nor IBM prescription drug insurance because the IBM enrollment period is close. However, you will have the ability to buy a Medicare approved prescription drug plan (and you should buy one to avoid a penalty even if you don't need it). You also MIGHT have the ability to buy a medigap plan for secondary insurance. It depends on the state laws whether or not the private insurance companies have to sell it to you.
Another change you can make any time during 2012 is to switch (one time) from your IBM Medicare Advantage plan to a "5 star" rated Medicare Advantage plan. Ditto for the Prescription Drug Plan. Once again, it is a change you can make that has nothing to do with IBM but once you make the change you cannot use IBM's offerings for the rest of 2012. The wrinkle in this option is there has to be a 5 star plan in your zip code. The way you find out if you have one is to go to plan finder on medicare.gov.
None of the above is relevant to people who are buying insurance for their non-medicare eligible spouses unless you no longer need to cover your spouse because once you switch, IBM will stop providing your spouse's insurance too.