Medicare supplements do not change with annual enrollment, only Medicare Advantage HMOs and stand-alone drug plans. Medicare supplements are defined by Medicare, offered by insurance companies. The plan remains the same no matter who the carrier is, but the premium can differ by carrier.
Here in California, we have the California birthday rule for Medicare supplement members. During the 30 days following your birthday, if another carrier for the same plan has a lower premium, you can elect to change to the new carrier in the 30 days following your birthday as a guaranteed issue (without having to go thru health history underwriting). I review this for my clients to let them know if there is any benefit to their changing their carrier in order to reduce their premium.
People on Medicare supplements should, during annual enrollment, review their drug plan to see if they are on the a stand-alone drug plan that will cover all of their drugs most cost-effectively.