All Medicare enrollees have an opportunity to either switch to Part C or add a Part D plan during their Initial Enrollment Period (IEP). This is a seven-month window that begins three months before your 65th birthday.
After the IEP, the only time to enroll in Part C or D is during the Annual Enrollment Period (October 15th-December 7th) or a Special Enrollment Period (SEP) under certain circumstances. (This blog has a more in-depth explanation of Medicare’s different enrollment periods).