Health and Human Services estimated that between 21%-43% of all people insured would transition to the ACA exchanges. This is due to their health care plans losing grandfather status.
Healthcare.gov, the official Affordable Care Act website, run by Health and Human Services, notes that:
Grandfathered plans are those that were in existence on March 23, 2010 and have stayed basically the same. But job-based grandfathered plans can enroll people after that date and still maintain their grandfathered status. [...] Grandfathered plans can lose their grandfathered status if they make certain significant changes that reduce benefits or increase costs to consumers.
What qualifies as a significant change that would cause an insurance plan to lose grandfather status?