During an enrollment period, if you already have a plan and sign up for a new one, you will be automatically disenrolled from the first plan. If you make multiple plan selections during a month, the last one you make will become effective on the first day of the following month.

However, you must be in an enrollment period to join a plan or change plans. If you try to do either outside of an enrollment period, you will be denied, unless you enroll under an approved special enrollment period.