Unless someone is newly eligible for a low income subsidy for their prescription drugs, it is only during Medicare’s Annual Enrollment (10/15 thru 12/7) that someone can select a different drug plan; however, that new drug plan will not be effective until January 1st of the upcoming year.
With you drug plan’s formulary in hand, contact your physician’s office to let them know that your new drug is not covered by your current drug plan and request other alternatives that may be covered this year by your current drug plan. Should the doctor indicate there is no alternative to the new medication s/he has put you on, see if s/he is able to provide you with samples.