If it's BCBSIL, they have a GI issue Plan G with no underwriting. Just submit a plan change request form to BCBSIL. Also - if both are on BCBSIL, you can apply for the 5% household discount (it was not available when Plan F was an option for all).
If they live in a zip code with Health Alliance, they offer GI medicare supplement plans as well to quote with BCBSIL to see which is cheapest.
If they live in a zip code with Health Alliance, they offer GI medicare supplement plans as well to quote with BCBSIL to see which is cheapest.