LOS ANGELES (MarketWatch) -- Health insurers said Thursday that President Obama's proposed change in the Affordable Care Act could "destabilize" the market and result in higher premiums. The trade group America's Health Insurance Plans released a statement from Chief Executive Karen Ignani, who said, "Premiums have already been set for next year based on an assumption of when consumers will be transitioning to the new marketplace." Obama offered to allow 2013 policies to remain in effect through 2014, under increasing criticism over his vow to let consumers keep their policies if they like them.