A new fight is brewing over health insurance companies letting millions of Americans renew their current coverage for another year -- and thereby avoid changes under the federal healthcare law.That may offer a short-term benefit for certain consumers and shield some of those individual policyholders from potentially steep rate increases. But critics say this maneuver could undermine government efforts to remake the insurance market next year and keep premiums affordable overall.At issue is a little-known loophole in President Barack Obama's landmark legislation that enables health insurers to extend existing policies for nearly all of 2014. This runs contrary to the widespread belief that all health insurance must immediately comply with new federal rules starting Jan. 1, when most provisions of the law take effect."Insurers are onto this, and the big question is how many will try to game the system," said Timothy Stoltzfus Jost, a law professor and health policy expert at Washington and Lee University.Some of the nation's biggest health insurers are looking to take advantage of this delay, and Arkansas officials are encouraging companies to do this by resetting customers' renewal dates for the end of December. There's also concern that some insurers and agents could rush to sell more individual policies before the year's end so they could be extended in 2014.Some policy experts are expressing concern about this practice for fear that insurers will focus on renewing younger and healthier policyholders and hold them out of the broader insurance pool next year. Their absence could leave a sicker and older population in new government insurance exchanges, driving up medical costs and premiums there."This could undermine the Affordable Care Act, and it opens the door for exacerbating potential rate shock in the exchanges," said Christine Monahan, a senior analyst at Georgetown University's Health Policy Institute. "The health insurers can cherry-pick some healthy people, and it raises prices for everyone else."This issue could affect some of the 15 million people nationwide who purchase their own coverage and millions more of the uninsured who are expected to join government exchanges next year. It would not pertain to the 150 million Americans who get health benefits through their employers.Many insurers are mulling over their options."Some carriers will require everyone to switch plans Jan. 1, and other carriers will allow customers to stay on their existing plan as long as possible," said Bob Hurley, senior vice president of carrier relations at the online site eHealthInsurance. "We are trying to nail this down with the carriers. I think it would be better for consumers to have that choice to carry their policy forward."The nation's largest health insurer, UnitedHealth Group of Minnetonka, Minn., said it's still looking at options.