Having the freedom to go wherever is certainly a very important aspect of the whole Medicare thing.
The following is taken from:
https://www.medicare.gov/suppl...
"When can I buy Medigap?
Buy a policy when you're first eligible
The best time to buy a Medigap policy is during your 6-month Medigap Open Enrollment Period. You generally will get better prices and more choices among policies. During that time you can buy any Medigap policy sold in your state, even if you have health problems. This period automatically starts the month you're 65 and enrolled in Medicare Part B (Medical Insurance), and it can't be changed or repeated. After this enrollment period, you may not be able to buy a Medigap policy. If you're able to buy one, it may cost more due to past or present health problems.
During open enrollment
Medigap insurance companies are generally allowed to use medical underwriting to decide whether to accept your application and how much to charge you for the Medigap policy. However, even if you have health problems, during your Medigap open enrollment period you can buy any policy the company sells for the same price as people with good health."
The best thing to do is not to wait 6 months if you want to get a supplement plan so you don't have to pass their health assessment.