I have a Lenovo P2 (42a) with Android 6, which I tried switching to Lineage 14.1, as my carrier hasn't provided any security/OS updates since 2016. I have previously done this with my old Motorola Moto G from Android 5.
I took a backup with adb (under linux mint), using the -system and -all options.
I unlocked the bootloader.
I flashed/booted TWRP 22.214.171.124 for this phone ("kuntao").
I transferred zip files. At one point the phone switched off while transferring with TWRP having mounted the "Internal Storage" partition, but it appears I got all things over on second attempt. It seems that sometimes TWRP gets stuck if the screen switches off while it is doing something, but the timeout can be increased if that is really the case.
I flashed the Lineage OS 14.1 file for the P2 ("kuntao").
I tried flashing the stock GApps zip, it let me know this was the version for 14.0 and I needed a different GApps file.
I then started over, flashing the Lineage OS 14.1 file and the GApps zip for 14.1 (Android 7.1). This would stop in various places, at least 3 different ones.
I then started over with the Lineage OS 14.1 file and the micro GApps zip for 14.1.
This completed, but when booting I get a black screen, a single long vibration and a continuously blue LED. The phone cannot switch off, only restart into bootloader or recovery, which at least isn't the worst news.
My adb backup does not restore - if I try this under TWRP, it seems to react to it, but get stuck, although with the screen on. When the command line process finishes, the result is the same as above.
Only issue I know of is that the provider can have added a few features, so that the flashing of the 14.1 file doesn't work exactly as intended. That would mean I need a complete image for Lineage OS or an image to restore to the "standard" Lenovo P2 state, as indicated here and here.
Any good ideas/procedures? Thank you.