[General Topics] urbanstarship:
I was able to downgrade from VollaOS 11 to 10 successfully, but it was quite difficult with the tools available. I gave up on MTKClient. For the benefit of others who want to do this, I’ll outline the steps I used. Special thanks for Volla Developer Community on Telegram for offering some suggestions.

I connected my Volla phone via USB to a Windows 10 PC and went through these steps.

1) Install Android SDK

2) Install MediaTek VCOM USB drivers

3) Download VollaOS 10

4) Allow USB debugging

5) Try ADB sideloading

6) Unlock bootloader

7) Allow USB debugging again

8) Install custom TWRP recovery

9) Remove device check from ZIP

10) Sideload OS from TWRP

11) Reboot the system from TWRP

1) Install Android SDK

You need two Command Line programs on your PC called ADB and Fastboot to make changes on your phone. These are included in the official Android SDK ‘Android Studio’ and are in the ‘platform-tools’ folder, which installed at this location by default:

C:\Users\user\AppData\Local\Android\Sdk\platform-tools

2) Install MediaTek VCOM USB drivers

The Volla phone’s CPU (a modified Gigabit GS290) is a MT6763V (aka Helio P23). Windows needs the right drivers to interact with a MediaTek based device. I found a set called "Windows 10 x64 MTK USB Drivers.inf”. Some sites will include installers with the drivers, but I installed “MTK USB Port” manually by selecting “Add legacy hardware” in the Device Manager. Lots of good YouTube tutorials how to do that step by step.

3) Download VollaOS 10

Get it from this repository here

Now that there’s 3 different Volla phones, be mindful of project names in the filenames that match your phone.

Volla = yggdrasil
Volla X = yggdrasilx
Volla 22 = mimameid

4) Allow USB debugging

First, unhide the Developer Menu on your phone. Navigate here and tap it 7 times:

Settings > About phone > Build number

A new menu will appear here:

Settings > System > Developer options

In this menu, turn on "USB debugging”. You may be prompted to allow it and trust the connected computer.

Now run the Command Line from the ‘platform-tools’ folder and check your devices are communicating

>adb devices

You should see a devices serial number

5) Try ADB sideloading

This is worth trying next, because the proceeding step wipes your phone completely. It didn’t work for me because of 2 errors:

Error 21: “Failed to verify whole signature”
E3004: "This package is for "[device]" devices; this is a “GS290”

Reboot the phone into maintenance mode by holding power + vol up

Select ‘recovery’ from the menu by pressing vol up, and confirm it with vol down

Select Advanced>Apply Update> from ADB

From the command line run:

>adb sideload volla-10.0-20220430-release-k63v2_64_bsp-signed.zip

This is where I got error 21. To get around this I needed to use the custom recovery TWRP, and to install that I had to unlock the bootloader, which wipes the entire phone.

6) Unlock bootloader

With the phone’s system running normally, reboot into fastboot from the command line on your PC:

>adb reboot bootloader

Then

>fastboot flashing unlock

Press volume up on your phone to confirm

7) Allow USB debugging again

Since your phone has done a factory reset you have to run through step 4 again to continue to run the debugging tools

8) Install custom TWRP recovery

Download a version of TWRP that works with Android 10, and put a copy in your platform-tools folder. I used twrp-3.5.0-10.0.img. Reboot into fastboot from the Command Line:

>adb reboot bootloader

Once the screen turns on with ‘Fastboot mode’ at the bottom, run:

>fastboot flash recovery twrp-3.5.0-10.0-GS290.img

If your PC reports all OK, then:

>fastboot reboot recovery

Your phone should reboot into TWRP.

9) Remove device check from ZIP

This is to get around Error 3004. Unzip your VollaOS package on your PC and find this file:

volla-10.0\META-INF\com\google\android\updater-script

Remove the following line and resave the file:

(!less_than_int(1651017058, getprop("ro.build.date.utc"))) || abort("E3003: Can't install this package (Tue Apr 26 23:50:58 UTC 2022) over newer build (" + getprop("ro.build.date") + ").");

Re-zip and then copy altered file onto a FAT32 formatted USB-C drive

10) Sideload OS from TWRP

Connect your USB Drive to your phone. Press ‘Install’ and choose your ZIP file on your USB.

11) Reboot the system from TWRP

When you reboot your phone it will prompt you to wipe your phone again because of corrupted files, but it should behave normally after that.

Note about TWRP: I tried for days to get TWRP to stay installed on the phone permanently, but it was always overridden by the Android 10 stock recovery when the system rebooted. I believe the way to circumvent this is to remove or rename the file /system/recovery-from-boot.p, but I couldn’t figure out how to get root permissions to alter this. One should be able to do this from TWRP, but the file remained hidden, nor did TWRP alter the ROM files as it was supposed to keep it installed. It’s annoying I can’t get it to stick, but I can load it again using step 8. Also, ‘fastboot boot twrp.img’ doesn’t work as it should either.