The Obama administration on Thursday plans to issue much-anticipated instructions for Americans to reenroll for next year in the new federal health insurance marketplace — a set of rules intended to make it easy for consumers but that nevertheless will require some people to reapply through HealthCare.gov to preserve their subsidies or coverage. Under the rules, people will need to do very little to remain in their health plans if their incomes and covered family members are not changing and their plans are offered again for 2015. A fraction of the people in the federal insurance exchange will need to reapply for one or more reasons.