This year, 37 states will use healthcare.gov as the website to enroll people through the marketplaces created by the health care law (PL 111-148, PL 111-152). The rest have their own websites. The sign-up period goes from Nov. 15 until Feb. 15.

Health and Human Services Secretary Sylvia Mathews Burwell has said that this year won’t be perfect, and that there will be times when the website is down, but federal officials expect things to go more smoothly than last year.

The new thing this year is that everyone who bought a marketplace plan for 2014 will have to re-enroll before Dec. 15. If they don’t, those in states that use healthcare.gov will be automatically re-enrolled with the same subsidy, to prevent their coverage ending on Dec. 31. Officials in some of the states that have their own websites have different policies.

HHS officials are urging people to log on and compare plans because a lot has changed since the previous enrollment season ended in April. Insurers are offering different plans in some states. The number of insurance companies offering coverage increased by about 25 percent, but some companies left the market or scaled back the number of plans they offer. The co-pays, deductibles and other cost-sharing will be different this year for some plans, as will the list of medicines that may be prescribed. The networks of providers also may be different.

One thing consumers who switch to a different plan or cancel their marketplace insurance should watch for is whether they might continue to be billed by their previous plan. Federal officials have not yet worked out a way to alert insurers if a customer cancels coverage, so insurers have to compare lists of customers to see which people leave and which ones join. That could cause problems if HHS officials don’t find a way to highlight cancellations for insurers.