Failing over from a previous version of SQL to a newer version of SQL is a one-way trip. You cannot failback from SQL 2014 to SQL 2008 since the database is upgraded once it becomes the principal on SQL 2014. The same concept applies to restoring databases between versions of SQL Server. You can go forward from 2008 to 2012 to 2014 but you cannot go backward from 2014 to 2012 to 2008.

This article explains in more detail:

http://msdn.microsoft.com/en-us/library/bb677181.aspx

Restrictions after you failover to an upgraded server instance.

After failing over from an earlier server instance to a SQL Server 2014 server instance, the database session is suspended. It cannot be resumed until the other partner has been upgraded. However, the principal server is still accepting connections and allowing data access and modifications on the principal database.