I success downgrade QC35II from 4.5.2 to 2.5.1.1182 then to 3.1.8.1835,and the decice work well, for ANC, it's hard to say.
7/19/2019 23:19:24.701, Error, Expecting version to be 4.5.2.144, read 2.5.1.1182
7/19/2019 23:19:24.701, Info, Update: Waiting for external bootmode reset -> Error state
7/19/2019 23:19:29.763, Info, Internal version is 2.5.1.1182
7/19/2019 23:19:29.763, Info, External version is 2.5.1.1182
7/19/2019 23:19:29.763, Info, Device version is 2.5.1.1182
7/20/2019 09:22:55.212, Error, Expecting version to be 4.5.2.144, read 3.1.8.1835
7/20/2019 09:22:55.212, Info, Update: Waiting for external bootmode reset -> Error state
7/20/2019 09:23:11.885, Info, Internal version is 3.1.8.1835
7/20/2019 09:23:11.886, Info, External version is 3.1.8.1835
7/20/2019 09:23:11.886, Info, Device version is 3.1.8.1835
but, i don't recommand to do it, high risk!!!!!!!!!!!!!! high risk, and high risk.
For windows:
When do updating, it will download the firmware to: C:\Users\<username>\AppData\Local\Temp
There are three files, something like:
Bose Updater.fl7980 33KB
Bose Updater.Nl7980 22089KB
Bose Updater.Ya7980 1897KB
Comparing these files by Beyond Compare 4 bytes by bytes.I find actully they are the 100% same files:
BayWolf_4.5.2_acorn_coeffs_signed.xuv 33KB
BayWolf_4.5.2_ext_signed.xuv 22089KB
BayWolf_4.5.2_stack_plus_app.dfu 1897KB
From the update log:
7/19/2019 23:13:51.661, Info, Update: Idle -> Downloading firmware
7/19/2019 23:13:53.846, Info, Done downloading https://downloads.bose.com/ced/baywolf/BayWolf_4.5.2_acorn_coeffs_signed.xuv (result NoError) (time 2)
7/19/2019 23:13:56.003, Info, Done downloading https://downloads.bose.com/ced/baywolf/BayWolf_4.5.2_stack_plus_app.dfu (result NoError) (time 4)
7/19/2019 23:14:01.714, Info, DIAGNOSTICS: Rate = 484528 B/s; Time = 50 sec.
7/19/2019 23:14:47.912, Info, Done downloading https://downloads.bose.com/ced/baywolf/BayWolf_4.5.2_ext_signed.xuv (result NoError) (time 56)
7/19/2019 23:14:49.799, Info, DOWNLOADING_FIRMWARE - DONE
7/19/2019 23:14:49.799, Info, Enter Internal Bootmode
7/19/2019 23:14:49.799, Info, Update: Downloading firmware -> Waiting for internal bootmode
7/19/2019 23:14:49.800, Info, reset-enter-internal-bootmode
7/19/2019 23:14:49.805, Info, Num excluded devices = 0
7/19/2019 23:14:49.807, Info, Waiting for device to drop out...(0x40fe)
7/19/2019 23:14:50.806, Info, Device has disconnected
7/19/2019 23:14:52.844, Info, waitForReset: Device is reset
7/19/2019 23:14:52.854, Info, waitForReset: Wait for device startup
7/19/2019 23:14:57.870, Info, 1 BOSE CONNECTED DEVICE(S) WITH VID 0x05A7
7/19/2019 23:14:57.870, Info, 1 \?\hid#vid_05a7&pid_4020#7&2cddf5a5&0&0000#{4d1e55b2-f16f-11cf-88cb-001111000030}
7/19/2019 23:14:57.972, Info, DeviceConnectionManager::doUpdate - DONE SUCCESS
7/19/2019 23:14:58.917, Info, Update: Waiting for internal bootmode -> Internal bootmode updating
7/19/2019 23:14:58.918, Info, HidDfuFlasher: Validating input file
7/19/2019 23:14:58.943, Info, Data to write {1894960}
7/19/2019 23:14:58.949, Info, Downloading firmware to device...
7/19/2019 23:16:06.480, Info, DoUpgrade: Data left to write {0}
7/19/2019 23:16:06.480, Info, DoUpgrade: retVal {0}
We can find, after finishing download image from the internet, it will reboot the devices to reset it. So the device will disconnect to PC. And when the device disconnect, there is a notification sound from windows.
So, If we hear such notification sound, copy the old firmware files to C:\Users\<username>\AppData\Local\Temp to cheat updater, then it will download the old firmware files to device and update.
Environment: Bose Updater ver is 6.0.0.4388, win7, chrome 75.0.3770.142
1.Go to http://btu.bose.com/ , enter "a","d","v","up","down"
2.Click update button.
3.Check the name of the three firmware files in C:\Users\<username>\AppData\Local\Temp They are looks like "Bose Updater.aaNNN". These files have different name everytime when you update and are removed by BOSE updater after you finish updating.
4.Check the size of the firmware files, match them to the old firmware files.
5.Rename the old firmware file to the matched one. For example:
Bose Updater.fl7980 33KB
match with
BayWolf_2.5.1_acorn_coeffs_signed.xuv 33KB
So rename "BayWolf_2.5.1_acorn_coeffs_signed.xuv" to "Bose Updater.fl7980". Follow such way and rename all these three firmware files.
6.Wait the first "device disconnection" notification sound.Since there are several "devices reboot", you may hear several "device disconnection" notification sound. Remember, the first sound.....
7.If there is a device disconnection notification sound,copy the "renamed" old firmware files to C:\Users\<username>\AppData\Local\Temp to replace the downloaded firmware files.Since there are only less than 2-5 seconds for replacing these "Bose Updater.aaNNN" firmware files.
7/19/2019 23:14:50.806, Info, Device has disconnected
7/19/2019 23:14:57.870, Info, 1 BOSE CONNECTED DEVICE(S) WITH VID 0x05A7
MUST MUST do it as fast as you hear the first device disconnection notification sound,otherwise it may brick/damage the device.You may re-update 4.5.2 several times to figure out when is the "first device disconnection notification sound".
8.Wait updater update the "cheated new firmware files"
9.When updating up to 100%, may show error message, just ignore it.
10.Check you device, the firmware is downgraded.
The old firmware files:
https://github.com/avicoder/Boss-headphones-firmware
Again, high risk!!!!!!!!!!!!!! i don't recommand you to do it, take you own risk!!!!!!!!! high risk.