Disability in December, Drop for a Supplement in January?

I have a referral that is going on disability before 65, with an effective date of her medicare supplement plan C being in December. This plan is provided by her retirement plan, and is overpriced. She wants to drop her employer plan in January to opt for a supplement plan F.

I am correct in assuming she can just drop her employer plan C and pick up an F on a GI basis in January, correct? The under 65 disability condition is throwing my rationality out the door for this one.

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