“The Obama administration is revamping HealthCare.gov and scrapping significant parts of the federal health-insurance marketplace in an effort to avoid the problems that plagued the site’s launch last fall, according to presentations to health insurers and interviews with government officials and contractors.

But the makeover—and the tight timeline to accomplish it—are raising concerns that consumers could face another rocky rollout this fall when they return to the site to choose health plans. Some key back-end functions,…”