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I want to reset my Lumia 640 to 8.1 in order to get wifi calling. But I have two questions about that:
Will Windows Update automatically update to Windows 10 whenever I update the phone, or will it just update to the latest 8.1 build?
There is no automatic updates from WP8.1 to W10M. You must install the Upgrade Advisor tool to upgrade to Windows 10 Mobile.
Just don't install the Upgrade Advisor. Turn ON automatic updates. You will receive updates for 8.1(if any) without installing Windows 10.
My phone was bought as an AT&T phone and they then gave me the unlock code. Will I need to unlock it again?
No need. Carrier unlocking is a one time process.
The unlock process is a one-time operation. Once the phone is unlocked, it will remain unlocked.
Once you update the phone to Windows 8.1, simply turn off automatic updates since it is highly unlikely any Windows 8.1 Mobile updates will be released in future.
I believe Windows Device Recovery Tool will automatically download the latest Windows Mobile 8.1 build to your device.
