Shopping to update your coverage on the Affordable Care Act's health insurance marketplace may be annoying — didn't you just do this last year? But letting the exchange automatically renew your coverage instead could be a big mistake. If you don't like the plan you're auto-enrolled in this year you may be stuck with it in 2018, unlike previous years when people could generally switch.

It's all in the timing. This year, the open enrollment period for the program, better known as Obamacare, will end on Dec. 15 in most states, including Pennsylvania and New Jersey. On Dec. 16, if you haven't picked a new plan, the marketplace will generally re-enroll you in the one you're in this year or another one with similar coverage.

But unlike previous years when the open enrollment period began in November as it did this year, but ran through the end of January, this year open enrollment through the federal exchange at will generally be over by the time you see which plan you've been assigned to.

"The deadline catches up with people," said Karen Pollitz, a senior fellow at the Kaiser Family Foundation. (Kaiser Health News is an editorially independent program of the foundation.)

Antoinette Kraus, director of the Pennsylvania Health Access Network, which offers free, in-person assistance to help people understand their options, worries that 2017 policy holders who didn't check their plan could find out too late that their physician networks are narrower. People who are eligible for premium subsidies might discover that by switching, they could have gotten a much better plan for less money.

"At the same time, people who are not getting tax credits" —  households with incomes above $49,000 for an individual, $98,000 for four — "are really freaked out because their rates are going up," Kraus said.

So it's important to affirmatively pick a plan to make sure you're getting the right coverage at the best price. Federal tax credits that help pay for premiums are pegged to low-cost silver plans, called benchmark plans. It's critical that each year you run the numbers to determine how your premium tax credit could be affected by changes in the benchmark plan and how that affects what you will pay out of pocket for coverage. Even if you are assigned to the same plan, your costs could be different next year.

Auto-enrollment doesn't take premiums or benchmark plan changes into account, said Stan Dorn, a senior fellow at Families USA, an advocacy group.

Last year, 2.8 million people were auto-enrolled. That's nearly a quarter of the 12.2 million people who enrolled in marketplace plans during the open enrollment period that ran from Nov. 1, 2016, through Jan. 31, 2017.

People whose plans will be discontinued by their insurer next year have a little room to maneuver if they're auto-enrolled on Dec. 16. Because their plan will no longer be offered, that's considered a loss of coverage, which triggers a 60-day special enrollment period. They can pick a plan through the end of February.

"But you have to pay the premium [on your existing plan], or you'll have a gap in coverage," said Pollitz, a risk that potentially exposes people to a penalty for not having insurance.

Kaiser Health News is a nonprofit news service covering health issues. It is an editorially independent program of the Kaiser Family Foundation that is not affiliated with Kaiser Permanente. Staff writer Don Sapatkin contributed to this report.