As you may already know since some time, Azure Active Directory Connect (AAD Connect) will be the only tool to synchronize On Premises Active Directory with Azure Active Directory (so replacing DirSync for Office 365) as well as providing more features.

So, a new preview build has been delivered for testing today. There is no upgrade path from the previous build.

Go to http://connect.microsoft.com/site1164/program8612 to get the new build