It may - depending on the state that you live in and if they have any added guaranteed issue periods legislated at the state level.

What state do you live in and I can see if that state has anything special about changing Medigap plans after your initial enrollment period. You can always talk to a SHIP representative or a Medicare Insurance broker. Many are independent agents that work with many different companies - UHC has captive agents - they only work with UHC plans.

Underwriting can apply anytime you are changing a Medigap plan - insurer or plan itself, once past that initial enrollment period unless your state has expanded this time period. I will also say that states that have done this also have higher rates as I am sure you understand.

Medicare.gov - Can I switch or Drop my Medigap policy?

It isn't just underwriting either - underwriting is when an insurance company will charge you a higher amount for a Medigap plan because you are outside of your Guaranteed issue (GI) period and have some health problems. But in many states, the insurer can also decline to cover a specific pre-existing condition for a specific period of time (state specified) or they may decline to cover you at all.

Medigap policies are not health insurance, they are more related to financial protection insurance.