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.