This simple shift in how healthcare is paid for—long seen as key to taming costs—has been occurring in pockets of the country. But nowhere is it happening more systematically than in Massachusetts, the state that blazed a trail in 2006 by guaranteeing its residents health insurance. Now Massachusetts, a model for President Obama's 2010 national healthcare law, may offer another template for national leaders looking to control health spending. "There have been few greater periods of change in American medical history ... and this is the epicenter," said Dr. Kevin Tabb, a former chief medical officer at Stanford Hospital and Clinics in Northern California who now heads Beth Israel Deaconess Medical Center, one of Boston's leading hospitals. "It is striking how different Massachusetts is from the rest of the nation."