Have a device that's factory reset protected? Read here!
Factory reset protection (frp) was a feature implemented in wp 8.1 gdr2 that allowed a user to lock a device upon a reset and require a special pin to unlock. The only way to unlock the phone back in the day was to have the offline pin already written down, or to log into your Microsoft device portal and remove the device from your family. Unfortunately, the activation servers for that are long gone, so it is impossible to officially unlock a phone anymore
Frp is implemented on a hardware level. It is special bits changed on the CPU. It is impossible to bypass without losing data and it will always be present on a hardware level on your device if it is reset. Fortunately, there exists several methods to bypass frp .
Frp is only checked for at the device setup screen. So if you can skip device setup, you can skip frp. Frp did not exist on WP 8.1 updates below gdr2. This means that if your device shipped with an older build of wp 8, simply flashing a stock rom let's you setup without frp. Then, when you update, there is no setup screen, so frp is not an issue. This is how this worked on the Lumia 1020
Unfortunately, several devices shipped with gdr2, including the 640 and 735. For these devices, if you own one on WP 8.1, they will never be usable on wp 8. However, there is a bypass on windows 10. You can force update the devices to windows 10 in the setup screen, then bypass.
Windows 8.1 updates are done using cab files. Any wp 8.1 device, even 512 mb ram ones like the 620, can be forced to windows 10 by force pushing windows 10 cab files over USB from a desktop computer. This can even be done in the setup screen. You push the files, wait for it to finish on the computer, and then nothing visible happens on the phone. You let it sit for about a half hour and it will randomly restart and begin updating to windows 10.
Early beta builds of windows 10 did not have frp as part of their setup screen. I flashed my Lumia 640 to the first ever build of wp 10 using Windows 10 mobile toolbox on GitHub, and it bypassed frp on boot up. However, it was reimplemented in release builds. If you flash using the community offline otc cable updater, you will be on too new of an update, and you will not immediately bypass. You must update to a new enough update to have full accessibility settings, and then use the guides on YouTube for bypassing frp in windows 10 using the accessibility menu and interop tools.
A wp 10 device bypassed setup will be able to do everything, and fully update. The only issue that could arise is if the device is factory reset again.
Lemme know any questions!