I just successfully flashed my K8+(2018) to oreo from nougat and kept my working fastboot. Im gonna tell you how but first........
I Duhjoker am in no way responsible for anything that may befall your device by trying any technique or by following this guide. Do this at your own risk.
What you will need::::::::
Lg K8+(2018) LM-X210ULM
K8+Oreo-rooted-boot.img =====second post
LGUP dual mode
Lg firmware extractor ??? This is for those portting a recovery and boot.img for a different cv1 device
Oreo firmware for your K8+(2018) X210ULM
Nougat firmware for your K8+(2018) X210ULM
Ok so go to settings and tap general then about phone then software and tap the build number 7 times and to make yourself a developer. Exit about phone and open your newly opened developers settings turn off auto updates and enable oem unlocking and debugging.
Now this guide assumes your cv1 device is on nougat. If you are already on oreo and have no ARB number and want to upgrade your oreo rom to include a working fastboot mode then you will need to first downgrade your cv1 device to the earliest version of nougat firmware you can find and repeat the steps above.
To unlock the bootloader go to your adb folder open up a command line and type
adb reboot bootloader
When it boots to a mostly black screen with a tiny lil paragraph describing fastboot and giving some instruction type
fastboot flashing unlock
fastboot oem unlocck
From there your done for now with fastboot. Lets go ahead and type fastboot reboot.
A developer here who was trying to unbrick his lg phone ended up hacking the program LGUP and found out a way to download to different partitions of the phones memory. The option called partition dl will allow us to flash a rom to the phone with out flashing the aboot partition which holds our working fastboot mode. You could even change your phone from a aristo 2 to a fortune 2 or k8 or any number of combinations as long as you do not ever flash aboot.
Warning! Warning! Warning! Warning! Warning!
Just a reminder. If you flash the aboot of anothet cv1 device to your phone it will immediately go dead and will be bricked under Qualcomm HSUSB 9008 mode which at this point in time there is no fix for.
Now you will need LGUP dual mode and since we are using a cv1 device go over to tecknights Aristo 2 page grab LGUP dual mode from there and thank him for all his hard work.
While your there if you want root you will need to go ahead and grab his twrp 4 cv1 devices with oreo kernel from his tutorial on flashing a prerooted oreo rom and port it over to the K8+ or not or i may have it posted here. Either way you will need it soon.
Now download the LG firmware extractor and pull the boot.bin from the oreo rom you are using to upgrade your device to and rename it to boot.img then put it in a dedicated folder in external storage.
Download the latest version of magisk manager put it on your external storage sd card with your oreo boot image.
Now lets flash our K8+(2018) X210ULM to Android 8.1 OREO.
Plug your phone into your windows pc and start LGUP dual mode Dev. When it loads your device hit select and the fladhing screen will appear with several more options than the LGUP your probably used to working with. Click the circle next to
Now go and choose the rom you want to flash and check every partition except aboot. Then hit ok. When it gets done it will reboot into Oreo and you will still have your boitloader is unlocked screen warning upon boot.
If not you did something wrong. Reflash nougat read the directions again and reflash the oreo rom with every partition except aboot. Remember do not flash aboot. If you do you will lose your working fastboot.
You now have a cv1 device that not only has fastboot mode but it can also flash.
Now for root go to settings and set up your developers options and this time you will notice that the oem unlock switch is greyed out in the unlock position.
Now install magisk manager and choose the install by picking a file and patching it. When it gets done you will find it in internal storage Download folder. Now hook plug your phone back into the pc and place your TWRP 4 CV1 DEVICES w OREO KERNEL and magisk patched image into the adb folder and rename them recovery.img and boot.img and open up a commandline there.
adb reboot bootloader
fastboot flash boot boot.img
fastboot flash recovery recovery.img
Upon booting go to magisk and let it setup up the enviroment and reboot then before rebooting to twrp go ahead and go to settings and Downloads in magisk and install TWRP helper. Then go do not reboot but go up to modules and hit the reboot to recovery option. It should now reboot to twrp where you can now mount your partitions then make a back up then check those mounts again and reboot to system.
Thats it. Your done.
This can be done with any cv1 device that does not have an ARB value on before you start flashing. You upgrade an Aristo 2 to Oreo as well as a Fortune 2. The only stipulation is that you choose firmware with an ARB value of 0 and start with a device with a firmware that has an ARB value of 0.
Ive heard that one could possibly change an Aristo 2 to a Fortune 2 and a Fortune 2 to a K8+ or vice versa but im not brave enough to try.
Please be sure to thank Tecknight for his work with the CV1 and you dont have thank me but it would nice if some one did.