 For some time, the annual enrollment period has lasted from October 15 to December 7 of each year but that is changing for Part C in 2019.In particular, low-income seniors on Social Security Extra Help/LIS and many middle-income seniors on state pharmaceutical assistance programs can choose a different plan or drop Part C/D more often than once a year.Medicare beneficiaries who were eligible for but did not enroll in a Part D when they were first eligible and later want to enroll, pay a late-enrollment penalty, basically a premium surtax, if they did not have acceptable coverage through another source such as an employer or the U.S. Veterans Administration.