Unless someone is on Medi-Cal or has relocated to a different region during the calendar year, you do not have a special election to change your plan coverage until the next Medicare Annual Enrollment (October 15 thru December 7) to be effective 1/1/2019.
Part of the enrollment packet you received for 2018 included a prescription drug formulary. The formulary lists the prescription drugs covered by your current plan. There is an alphabetical list at the back of your prescription drug formulary. Anyone on Medicare should take their drug formulary with them to all medical appointments or have it by the phone if they are expecting a call from their doctor so that they can confirm with the doctor whether or not the drug being suggested is covered by your current plan whether it is one of the higher tier/more expensive drug. You can ask the doctor if s/he has any samples for your to try or if there is an alternative prescription drug that might be an option for you that will be covered by your current drug plan formulary.