SIM-Unlocked Sprint Moto X won't update beyond Jelly Bean, how can I update?

[AC Question]

As the title says, I bought a new and SIM-Unlocked Moto X from a vendor (phone was originally a Sprint device), and when I try to update the software to presumably KitKat, they phone says I'm "Up-To-Date". The phone's settings show that I'm still on 4.2.2, and even the Motorola Device Manager on my home PC says that there are no updates available. What's going on here? I know Sprint has 4.4.4 for this nearly a year ago.

Please advise on how to update