SEP Question when Moving to Another State

I have a client who is 70 and has been on an MAPD plan for 5 years. In July he is moving to another state. Here is the question: he does NOT want to enroll in another MAPD in his new state. He wants to switch to a med supp. So when someone is on an MAPD and they move to another state, do they have to stay with MAPD or can they switch to med supp?

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