(Bloomberg) — Americans can save money by shopping for new health insurance plans through the Patient Protection and Affordable Care Act (PPACA), U.S. officials said, in a push to keep people from getting surprised by bigger bills when the coverage they have now through the program renews automatically.

Of the 765,135 people who applied for insurance during the two weeks starting Nov. 15, about half were customers who had coverage this year through the marketplaces created under PPACA, which is also known as Obamacare, As a result, millions of people will be renewed in their current plans. The deadline to change plans or pick a new one is Dec. 15 for coverage that starts in January.

Not switching could be costly, officials say.

Some observers say many consumers may prefer sticking with a health insurer that seems to have good service and a good provider network than saving a little money by switching to plans they know nothing about.

But about 70 percent of the people who have insurance through the marketplaces can get the same level of coverage for less if they shop around, the U.S. Department of Health and Human Services said today in a report. If all returning customers bargain-shopped for a new plan, they could save more than $2 billion on premiums, according to the report.

Bills for people who let their plans auto-renew are likely on the way now and may upset those who didn’t look for a better deal, said Bob Laszewski, an insurance industry consultant in Alexandria, Va.

“These premium statements are about to arrive and people are going to hit the roof,” he said.

Health officials stressed it will take people only 30 minutes to re-enroll in plans. The U.S. report included data from 35 states and doesn’t include those that run their own insurance markets.

“It’s so important for our customers to come back and shop and see what their options are,” Kevin Counihan, chief executive officer of the U.S. marketplace, healthcare.gov, said on a call with reporters today. “We strongly strongly encourage people to come back to the website and shop.”

The administration’s goal this year is to cover 9.1 million people next year through the state and federal program, compared with 6.7 million as of October.

See also: 5 exchange plan re-enrollment stories.

Copyright 2018 Bloomberg. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.