"I'm on Medicare Part A and B. I want to drop Part B and buy a health plan through the Exchange so that I can get a subsidy"

Don't do it.

That's worth saying again: Do Not Do This.

Here's why: Most health insurance plans have language in their policies that lets them drop anyone who is eligible for Medicare. As a result, even if you manage to sign up for the plan, the company will likely eventually figure out that you're eligible for Medicare and will drop you.

Then, if you go back onto Medicare Part B, you'll have to pay a penalty for as long as you continue to have Medicare. The penalty is 10 percent for each full 12-month period that you could have had Part B.

And that's not all. If you are Medicare-eligible and you purchase a plan offered on the Exchange, you are not eligible for an Exchange plan subsidy. (If you are on Medicare, you are already getting a subsidy, because the federal government pays far more in Medicare costs that current Medicare recipients paid into the program.)