WASHINGTON — The federal agency overseeing the Affordable Care Act insurance marketplaces started sending re-enrollment notices Tuesday night to the millions of people who purchased new coverage this year. And these consumers won’t have a lot of time to act if they want to get a better deal on health insurance starting in 2015.

The next open-enrollment period starts Nov. 15 and runs through Feb. 15. But individual consumers enrolled in Affordable Care Act health plans will have until just Dec. 15 to pick a new plan and be enrolled by Jan. 1. Otherwise, they’ll be automatically re-enrolled in their plan, which could cost them more in 2015.

About 7.3 million people were enrolled in marketplace health plans as of mid-August, according to the latest count from the Centers for Medicare and Medicaid Services, the agency running the HealthCare.gov portal in 36 states.

Shopping for health insurance isn’t exactly the easiest thing to do, and inertia tends to keep most people in their existing coverage.

CMS officials said they have made the HealthCare.gov shopping process easier for this coming enrollment period. People coming back to the site this year will have their applications automatically filled out, for example.

This is the first time the marketplaces have had to handle re-enrollment, which presents new issues for the government and the industry.