Many in New York’s professional and cultural elite have long supported President Obama’s health care plan. But now, to their surprise, thousands of writers, opera singers, music teachers, photographers, doctors, lawyers and others are learning that their health insurance plans are being canceled and they may have to pay more to get comparable coverage, if they can find it.

They are part of an unusual, informal health insurance system that has developed in New York, in which independent practitioners were able to get lower insurance rates through group plans, typically set up by their professional associations or chambers of commerce. That allowed them to avoid the sky-high rates in New York’s individual insurance market, historically among the most expensive in the country.

But under the Affordable Care Act, they will be treated as individuals, responsible for their own insurance policies. For many of them, that is likely to mean they will no longer have access to a wide network of doctors and a range of plans tailored to their needs. And many of them are finding that if they want to keep their premiums from rising, they will have to accept higher deductible and co-pay costs or inferior coverage.

“I couldn’t sleep because of it,” said Barbara Meinwald, a solo practitioner lawyer in Manhattan.

Ms. Meinwald, 61, has been paying $10,000 a year for her insurance through the New York City Bar. A broker told her that a new temporary plan with fewer doctors would cost $5,000 more, after factoring in the cost of her medications.