Did it come from AT&T with Windows 10 already installed?
I have AT&T Lumia 640 XL LTE (purchased from Microsoft Store), and the Update Advisor app says:
"Good news! Your phone is eligible for upgrade to Window 10. We are working with your mobile operator and phone manufacturer to make the upgrade available for your phone and you'll see a notification when it is ready."
While Windows Phone 8.1 works great on this phone, the Bank of America app is available for Windows 10 Mobile, but not Windows Phone 8.1.
This is true for a couple of other apps I would like to use also.
Lilla