"Please Note: Beginning July 1st 2015, users who have not used their Flex transponder for HOV transactions on Express Lanes for any prior six month period will be required to exchange their Flex transponder for a standard transponder (at no cost) or be subject to a one-time transponder functionality upgrade fee of $10. This only applies to transponders issued after October 1st 2014."
http://www.ezpassva.com/StaticPages/FAQ.aspx#four
Read #2 under the FAQ