Once I join a Medicare Advantage Plan, will the co-pay amounts always remain the same?

No. The administrators of the Plan can change co-pay amounts, pre-certification guidelines, and other policies from time to time, often on an annual basis. They may also change their network providers so that a physician might be on the plan one year and off the next, or vice versa, or off the plan one year and on the plan the following year. Usually the provider group stays about the same but there are some changes. Co-pay amounts often change annually.