I received the email from F9 on Feb 4, 2015. All my miles are still there as of today.

"Changes to EarlyReturnsŪ Mileage Expiration
Beginning March 7, 2015, you'll need to earn miles using your EarlyReturns account every six months in order to keep the miles that are currently in your account.

The good news, your miles don't need to expire! Just keep earning and you can keep them. Any miles you have earned, or continue to earn, will not expire as long as you continue to earn miles every six months. Unredeemed mileage accumulated in your account will expire at the end of the six-month period after your last activity date. For example, if you don't acquire new miles by September 7, 2015, the miles you've accrued through March 6, 2015 will expire on September 7, 2015 and won't be available for use."