Barbara Gruber has never been happier to pay a bill.

From CareFirst BlueCross BlueShield, the bill confirmed that the 55-year-old Mount Washington woman had new, affordable health insurance. She bought it on the Maryland Health Connection, the online exchange for the uninsured that was created to comply with the federal Affordable Care Act.

But Gruber, like many buying insurance on the exchange, was not without coverage before. "I'm a heart patient and need insurance. ... I can't be without it."

The state cannot say how many Marylanders were switching from existing policies when they bought insurance on the exchange. Some, like Gruber, simply moved to cheaper or more comprehensive plans — even though the main goal of the law, known as Obamacare, is to cover most of the uninsured.

That Gruber already had insurance did not preclude the state from counting her among the 22,512 people who have bought private coverage on the exchange so far, and toward the state's goal of 150,000 during open enrollment through March.

Such buyers likely make up a "significant number" of those signing up on state and federal exchanges now, said Matt Buettgens, a senior research analyst at the Urban Institute, a nonpartisan research group.

"They aren't likely to be the majority in the long run," he said. "But they're getting renewal notices in the mail, and they aren't happy with their current insurance or they're losing their insurance, so they may be the first ones to act."

Maryland's exchange, which has been troubled by persistent technical problems, does not track the number of users who already had insurance. Neither does the federal exchange, which has enrolled 1.2 million people across 36 states in private plans.

Peter Morici, a University of Maryland economist, said that casts doubt on all of the enrollment numbers. "All the data coming out of the exchanges is so loose," he said. "I don't think at this time we get a clear picture of how many people actually have been added to the rolls."

Maryland's health secretary, Dr. Joshua M. Sharfstein, said it's not surprising — or unwelcome — that many on the state exchange are not newly insured but are trading up.

"It's a good thing people can now qualify for subsidies and pay less," Sharfstein said. "We want to see people in better-quality coverage that's more affordable and stable."

That's what Joann Shervington of Columbia said she got by replacing her policy on the health exchange.

She spent about a year and a half without insurance after she left her job in 2009 in the insurance industry to care for her ailing husband. She focused attention on him and made sure he had health care while he was alive. She eventually enrolled herself in a limited program offered to Howard County residents called Healthy Howard.

Notified that the coverage would end when the state exchange opened, Shervington began researching her options. When the website gave her trouble, she signed up directly with Evergreen Health Co-op, founded by Dr. Peter Beilenson, a former Baltimore health commissioner who also launched Healthy Howard.

At 60, she's pleased to have more access to specialists who can help keep her diabetes under control and prevent new problems.

"I have enough medicine to last until next month," Shervington said. "I wanted to maintain consistency in coverage. No gaps. ... I want to make sure everything is in control."

Meanwhile, there is no hope of reaching all of Maryland's 800,000 uninsured. About half are not legal residents and do not qualify for coverage.

The Hilltop Institute at the University of Maryland, Baltimore County estimated before the health law launched that by 2020 about 6.3 percent of the state's population would remain uninsured, down from just over 10 percent in 2014.

Of 145,500 Marylanders enrolled in private insurance plans or in Medicaid so far, almost two-thirds had been covered under a bare-bones program for low-income adults before being automatically moved to Medicaid.

Underscoring the challenges of reaching the uninsured, a recent Gallup poll found that nearly three-quarters were not familiar with the Affordable Care Act. And the Maryland exchange's technical troubles might have put off some potential customers, particularly those who were without insurance.

Buettgens said outreach campaigns will make a dent in the uninsured. For example, major tax preparers plan to help determine their clients' eligibility for Medicaid or subsidies and steer them to enrollment assistance.

Maryland has its own marketing effort underway, and surveys will show whether they are effective, Sharfstein said.

That stability, however, isn't assured over time, potentially undermining efforts to insure more people. The bulk of Americans get coverage through employers, but people who frequently change or lose jobs or are self-employed often bounce in and out of coverage, Morici said.

He said studies have shown a lot of that involuntary movement, called churn. Hilltop, for example, found among Medicaid beneficiaries in Maryland in 2011 that about 20 percent were new to the program and 13 percent lost eligibility.

Buettgens expects churn to rise because there are more people eligible for Medicaid and subsidies. Officials estimate that close to one-third of people in any given year could gain or lose their eligibility for aid because of changes in employment or family status, making programs more costly to administer and interrupting coverage.

He said states and insurers were looking at ways to offer continuous coverage. One example is Evergreen, one of the insurers on the Maryland exchange. It has signed an agreement with a Medicaid manager that allows Evergreen customers who become eligible for Medicaid to keep their Evergreen plan.

Janet and Bob Anderson of Timonium spent time without coverage after moving from western New York to be closer to their children more than a year ago. They waited for Maryland's exchange to open to shop for new policies.

She said it was a "second job" investigating their insurance options on the exchange, qualifying for subsidies and signing up for a plan. But they are glad for the insurance they finally got the day after Christmas.

Janet Anderson, who runs a housecleaning business, acknowledged that she and her husband, a building contractor, "were taking a chance. I felt so relieved to get coverage because anything can happen at any time. This is one more thing off my shoulders."

Gruber, who works as an adjunct professor at several area universities, is elated with her new coverage.

But she said those using the exchange must be persistent. She was stymied for weeks after the site's Oct. 1 launch by technical glitches online and busy signals at the call center, but used Facebook to contact a "navigator" at HealthCare Access Maryland for guidance.

She was desperate to reduce her $520-a-month premium plus $300 for medications. She qualified for federal subsides on the exchange and will now pay $140 a month for medical and dental coverage.

"Is this better? Are you kidding me?" said Gruber. "A third of my income was going to medical expenses. I'm up to my ears in hock."

meredith.cohn@baltsun.com