The Missouri Anniversary Rule allows a person with a Medigap Supplement plan to price shop that plan without having to answer health questions ~ it is Guaranteed Issue ("G I").

30 days before and 30 days after the annual anniversary date or "Effective Date" of your Medigap plan you can change to an insurance carrier offering the same Medigap plan at a lower premium.  The rule is "like to like".  So, if you have a Plan F you can shop for another carrier's Plan F; or Plan G to Plan G or Plan N to Plan N et cetera.  Letter to letter.

In addition to filling out the new company's application for insurance, you will need to provide proof to the new company that you already have a Medigap plan.  This is done with a copy of your Medigap Supplement card if it has the Plan Letter on it (like F or G or whatever) and the Effective Date of that coverage.  Or, better yet, a copy of your "Policy Schedule" page, which typically, can be found at the back of your Policy.  This page might be called a "Declaration Page" or some other such name.

To Recap and Summarize: 

Documentation with your name,  Medigap plan letter (like G), your current Medigap plan's Effective Date and the Policy # of your current Medigap plan.  Copies of these things will be needed to include with an application for the new insurance company to verify that you have a Medigap plan in-force now.

(Wheew  Lot 'O wordz.)  K.W.