Medicare Advantage plans are booming — 30.8 million of the 60 million Americans with Medicare are now enrolled in the private plans rather than the traditional government-run program. But a little-known fact: Once you’re in a Medicare Advantage plan, you may not be able to get out. Traditional Medicare usually requires beneficiaries to pay 20 percent of their medical bills after their deductibles are met — a potentially ruinous expense that most people cover in part with a private supplemental plan called Medigap. But unless you sign up for Medigap soon after you’re first eligible, insurers ca…