I don’t know if this post will be useful to many OnePlus 5T users who started with the ‘o’ version (Android 8) of /e/OS, which is my case, but I thought I’d just add a few steps and modifications to the official upgrade guide on the documentation site, as it may help others avoid the difficulties I had.
Download - for the TWRP recovery; do NOT use the 188.8.131.52 build which the link points to as it ends in “error 7”.
Use this link to get the 184.108.40.206 version as modified by codeworkx.
If you already have the 220.127.116.11 version on your device simply re-flash the downgraded version using this command:
fastboot flash recovery twrp-3.3.1-xxx-dumpling.img
Or use another version by mauronofrio or blu-spark.
Download the latest firmware version 10.0.1 available here and either use adb to sideload the zip file or copy to internal memory and flash the firmware upgrade using your new TWRP recovery.
Reboot into your present /e/OS and remove any PIN or swipe protection you may have. Then as the e-upgrade doumentation says backup any presonal data on your phone including SMS and call logs if you want to restore these on your upgraded phone. If you have an ecloud account and have synced your data and contacts this simpllifies matters as they will be re-synced and restored when you log in to your account after the upgrade. I personally used Titanium Backup to be on the safe side but this requires a rooted phone.
At this point you can re-boot into TWRP recovery and proceed with a factory reset (wiping the /data, /system and /cache partitions) then sideload the latest official Pie version for dumpling as described from steps 11 to 15.
Hope this is useful.
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