What are the upgrade options for an unlocked AT&T Galaxy S4

[AC Question]

I have an unlocked AT&T Galaxy s4. In its heart of hearts it still thinks it's an AT&T phone. That's where the update manager looks for Android updates (which it never finds). The stock contacts app tries to contact AT&T too. What are my options to make it forget all that stuff? It's currently running 4.4.4 i337ucufnj4.