Congress can pass a law if they want. The insurance companies will not be able to un-cancel those policies. That ship has sailed. Too many people will have moved on. The insurance cannot reassemble the risk pools deastroyed by 0bamacare. The actuarial assumptions of underwriting the risk will have changed too much to just renew those policies.
Not to mention the regulatory hoops the plans would have to go through with a slow-moving bureauocracy.