As of April, 2017 United Health Care will no longer offer health plans through the ACA Marketplace.  Since the enrollment period will have ended by that time, subscribers will be able to keep any plans that they have enrolled in during the 2017 enrollment period.

United Healthcare, the biggest health insurer in the United States, said Tuesday 
that it plans to exit most of the Affordable Care Act state exchanges where it 
currently operates by 2017. The health insurer had already indicated that it was 
dropping coverage of the plans, more commonly known as Obamacare, in Arkansas, 
Georgia and Michigan. But during a conference call with analysts Tuesday, 
CEO Stephen Hemsley noted that "next year we will remain in only a handful 
of states."