When changing to a new Medicare plan, the Social Security Administration sometimes needs several months to update their records with the correct Medicare Part D premium deduction information.
When the Social Security Administration has updated their records, you will see the erroneous $40 monthly premium deduction credited back to your account and the actual $15 premium amount deducted.
For more information, you can always call 1-800-772-1213 and speak with a Social Security representative.