If you’re already Medicare age or past it, stick with what you have if you’ve got medical problems. Any new company is going to review your health condition/problems and adjust the premium accordingly.
Not during open enrollment periods unless changing from Advantage plan to supplement (good reason not to join a Part C Medicare Advantage plan because you're basically stuck there if have medical conditions).
If you want to switch Medigap plans, or you’re joining from a Medicare Advantage plan or original Medicare without supplemental coverage and you’re not eligible for open enrollment, then you’ll probably face underwriting.