This is a video I did recently on Medicare Supplements and Pre-Existing conditions.
I lost a client once because one of the Medicare Supplement companies, CIGNA, sent out their new policy with a highlight on a Pre-Existing Condition warning. They did not, however, explain in their materials that the pre-ex does not apply if they've already had their Medicare Supplement policy with their former carrier for at least six months.
So, if you are replacing one Medigap (Medicare Supplement) policy with another one, and they were with the old company for at least six months, there are no pre-existing condition exclusions. There are no waiting periods, there are not denied claims with the new carrier where that would not have been denied with the old carrier (if they're moving to the same plan letter with the new carrier).
Here is the video explaining how pre-existing conditions are treated when changing Medicare Supplement policies.
Here is the “Choosing a Medigap Policy” guide:
http://www.medicare.gov/pubs/pdf/02110.pdf
You can request copies of this from Medicare directly (www.medicare.gov) or by requesting it from any insurance carrier you are writing business for now.