Medicare has grown into being one of the nation's largest health insurers - some 55 million of us are in it, mainly seniors, and about one in four now buys a so-called Medigap policy, aka Medicare Supplement, which is intended to pick up costs that would otherwise slip through the coverage cracks of Medicare.

What you don't know about Medigap when you buy that policy may come back to bite you.

There are ten Medigap plans commonly available, and labeled "A through N. Coverage" - and prices - vary wildly from plan to plan. A high deductible Plan F might cost $50 per month; there's a $2,180 annual deductible. Standard Plan F - one of the most popular plans - might cost $170 per month. It covers foreign emergency care (usually ignored in most Medigap plans), care in a skilled nursing facility (not covered by some Medigap plans) and a lot more. Medicare succinctly outlines plan differences here.

Here's the problem, and it is understood by few who enroll in Medicare: start out with a bargain Medigap plan, because you want to save the money and, furthermore, your health is good, and you may think that some years down the road, when the heath has worsened, that it will be a matter of checking some boxes to transfer into a higher coverage plan when you need it. Not so much.

"At 65, when you enroll, you have guaranteed issue," said Brooke Thomas, director of Medicare at eHealthMedicare.com. "You don't have it later." At 65, the typical Medicare enrollment age, you pick your policy, fill out some papers, and the federal government guarantees you will be accepted and also that you will not be hit with a surcharged premium because of your health.

Choose that initial plan carefully, because very probably you are stuck with it. 

Once six months have elapsed, those guarantees to change without bother vanish. In most states, a Medicare beneficiary who later seeks to change Medigap policies will be subjected to what the industry calls "medical underwriting."

"I get questions about this all the time," said Ash Toumayants, president at wealth management firm Strong Tower Associates in State College, Pa.. Toumayants said that if your present insurer is leaving the business, or if you have decided to quit Medigap and enroll in Medicare's managed care alternative, Medicare Advantage, you will have smooth sailing.

If you want to upgrade your coverage - maybe you have decided to travel abroad a lot and want one of the rare Medigap plans that cover foreign emergencies - good luck with that. Toumayants said that what happens is that once you initiate a request for coverage you are given a lengthy health questionnaire to fill out. Different insurers have different rules. He added: "You have to be able to say no to all the questions. If you say yes you can be rated or declined."

Big money can be at stake. Thomas said a healthy 70-year-old might pay $170 monthly for Plan F. But he has seen quotes as high as $350 for seniors with health issues.

Comparatively small health issues - such as controlled hypertension or high cholesterol that is controlled with medication - can trigger premium upcharges. Bigger issues - such as saying "yes" to diabetes - may trigger instant denial, said Toumayants. He added that he has seen some insurers say no to anybody with diabetes. Others say yes if the disease is controlled with orally administered medicine. But use a needle, and and that is a rejection.

Also know that different states have different rules, said Nate Purpura, vice president of consumer affairs at eHealth.

Adria Goldman Gross of MedWise Insurance Advocacy in New York said that in that state Medicare recipients can change Medigap policies without medical underwriting.

California has a similar rule that applies, a so called birthday rule, that allows residents to change Medigap coverage at their birthday, without medical underwriting, if they are changing to a plan with fewer benefits.

Thomas said that in most other states, if you are changing policies with the same insurer, you often can downgrade coverage - typically motivated by a desire to save money - without medical underwriting. There's no guarantee of that, however.

Confused? You have every right to be. That's why advice from multiple sources is that when contemplating a Medigap plan switch, work with a broker who is a Medicare expert.  You may be able to change Medigap plans. Perhaps even without paying surcharged premiums. But some insurers may reject you outright, while others may take you. A good broker should know. That will make the hunt go a lot faster.

This article is commentary by an independent contributor. At the time of publication, the author held TK positions in the stocks mentioned.