During your initial Medicare enrollment period you are guaranteed issue any Medigap plan available in your area. Plan N is an excellent choice at this time. After the enrollment period ends, you are generally subject to medical underwriting in most states if you want to change.
There are certain other situations which generate a Special Enrollment Period. With a SEP, you are guaranteed the right to purchase Medigap Plans A, B, C, F, K, or L. So, if you are healthy you can go through the medical underwriting process for Plan N. But those unable to pass underwriting have to choose a guaranteed one such as Plan A.
Guaranteed Issue:
https://www.medicare.gov/supplement...eed-issue-rights-scenarios.html#collapse-4690