SQL Server AG databases went into 'Synchronizing/In recovery' after upgrading to 2014
The Availability Group prerequisites all indicate that you must be using Enterprise Edition and be using the same version of SQL Server.

See the MSDN link for SQL Server 2012 

https://msdn.microsoft.com/en-us/library/ff878487(v=sql.110).aspx

Under the topic:  Checklist: Prerequisites (Server Instance) it says:


  Each server instance must be running the Enterprise Edition of SQL
  Server 2012.


Likewise 2014 and 2016 have the same prerequisites for only those versions.

While my response was focused on the requirement in general as noted in MSDN. However, Kin in his comment added details on upgrade path: 

"In OP's case, he was upgrading secondary to a higher version which should be fine (once failover is initiated, it will be one way ONLY - no role changes are allowed), and then he might upgrade the primary to 2014. This is how upgrades should be done."