The Department of Veterans Affairs on Thursday will begin allowing a broad section of its nine million enrollees to seek medical care outside of traditional V.A. hospitals, the biggest shift in the American health care system since the passage of the Affordable Care Act nearly a decade ago.

While department officials say they are ready, veterans groups and lawmakers on Capitol Hill have expressed concerns about the V.A., which has been dogged for years by problems with its computer systems. They worry that the department is not fully prepared to begin its new policy, which Congress adopted last year to streamline and expand the way veterans get care.