Fyi at least for QC 35 I downgrading does not help

[krackers]

It should be noted that at least for my qc35 downgrading from 3.0.3 to any lower firmware did not restore NC to what it used to be. This of course sounds insane, but several others on reddit seem to have corroborated. Given that FW updates and device resets don't affect the paired device list, my best guess is that there's some region of memory that was touched by the 3.0.3 update that isn't reverted back when downgrading.

The firmware themselves seem to be for BlueCore IC, but no idea if anyone has ever RE'd them

[krackers]

See this section

As well as containing the firmware image (i.e. control software), firmware files can also contain configuration
information known as Persistent Store (PS). DFU files can contain information for a partial or full update of the PS.
Any areas of the PS that are not in the DFU file will remain as they were before the update. Other firmware files
cannot contain partial information; they either preserve or replace the PS.
Firmware upgrades from the CSR support website do not contain any PS settings, thus the current settings are
preserved. Firmware dumps that you make contain PS settings, and so will overwrite any existing ones if you upgrade
using a previously dumped file

[krackers]

Maybe this is placebo... compared the qc35 I upgraded to 3.0.3 and downgraded to 1.3.2 against a qc35 II at 4.1.3 that was never upgraded. Everything seems same between them. I could have almost sworn that originally there was a difference between high/low NC modes in a silent room (since I remember testing this when writing cli app to send packets to trigger the nc switch), but maybe my ears got used to it.

[tredin8r]

Hey how did you get them downgraded with the new Bose Updater patch? Sick as hell, please let me know!

[krackers]

see #33, basically use charles or some other mitm to intercept the xml

[ItxWeird]

Hey, reccently downgraded to 2.5.3 and I found this to be the best firmware for noise cancelling

[Ibamezz]

see #33, basically use charles or some other mitm to intercept the xml

i have tried with charles, but it wont help if both worldwide.bose.com and downloads.bose.com points to this lookup file i cant see any lower version than 2.5.5, how do you really manage to do it?
I tried this a long time ago when the old method worked with modified exe.
But now no luck at all. Sometimes i even get that multiple sessions are used.
An updated guide for Charles would be great, and much appreciated.
it is on Windows 11 latest updates i have tried. i don't have access to any other operating system.