[HTC U11  Questions & Answers] tejHackerDev:
If You want to downgrade from Pie to Oreo. Its pretty easy and simple you has to follow same steps like Oreo to nougat downgrade but with some modifications:- (bootloader unlock is required)

1. I tried it on my own device it worked and it is s-on, So i don't know it works on s-off or not.

2. Download HTC Dev info tool from the below link

https://drive.google.com/file/d/1ubjDGNqCfIply-aRN8C7IQNb-YVkD4M7/view

3. Now connect your device to PC by turning on USB debugging in phone and open the info tool and click on GetDeviceInfo and it will show your device information in the tool.

4. If you want to change CID then follow this step otherwise skip to step 5. Change the CID to your desired CID and click on save.

5. Now change the misc version to the rom misc version which you downloaded for downgrade and click on save and apply changes.

6. Now download and install minimal adb and fastboot.exe in PC and open it and type "adb reboot download" and your phone boot to download mode and type "fastboot getvar all" and it will print your device information in the cmd itself and from their cross check your CID number whether it is changed to your required CID or not and also check the version number is blanked out or not. If it doesn't then something wrong you have done, so again start from step 2.

7. Download the Oreo RUU for your CID from the below link :-

https://docs.google.com/spreadsheets/d/15JL3tRWDSVOUKo_revEYtQ_tRcOSJ0vWcvk8a7TK8Hk/edit?usp=sharing

8. Now Download BatchTool.exe from the provided link below

https://xdaforums.com/showpost.php?p=67056758&postcount=4

9. Install the tool and and go to the location where you installed the tool generally it will be c:/Android/com now copy the RUU you downloaded for your CID to that folder and rename it to rom.zip for simplicity ie., c:/Android/com/rom.zip

10. Now Download RUU decrypt tool from the below link and extract the zip and drag and drop the rom.zip on to the RUU_decrypt_tool.exe present in the extracted folder.

https://xdaforums.com/devdb/project/?id=15338#downloads

11. And An CMD will pop-up and ask for some yes and no details just accept the default settings with out changing anything and now the RUU decryption will start and after completing a folder will be created in the RUU decrypt tool folder and inside OUT folder look for "hosd_signed.img" and move it to the rom.zip folder ie., c:/Android/com/hosd_signed.img

12. Now open cmd and navigate to the com folder by using by using cd command.

13. Now cross-check whether you are in bootloader mode or not, if yes then proceed to next step otherwise reboot your phone to bootloader mode by typing "htc_fastboot reboot bootloader" (htc_fasboot is a file present in com folder so you need to be in com folder to execute that commands from cmd).

14. Now type "htc_fastboot flash hosd hosd_signed.img" and wait for it to flash and after you see written successful then reboot to RUU mode by typing "htc_fastboot oem rebootRUU" l

15. On booting your phone to RUU mode type "htc_fastboot flash zip rom.zip" (rom.zip is file name of your RUU that you downloaded and renamed to rom.zip).

16. Now the flashing process should start and wait for some time until it finishes.

17. Now you have successful Downgraded from PIE to OREO with your desired CID..,.,.,.,.,:fingers-crossed::fingers-crossed::D