[OnePlus 9 Pro] craznazn:
*** Use this at your very own risk.

*** You will lose all data

*** Backup your working modemst1/2 and persist. If you lose these for any reason, it's unique to your device and not recoverable if you didn't back up (via MSM or other people).

*** Read the entire post and follow the steps closely before asking for help. Post all error messages and steps you took. If you just ran a bunch of unknown commands, I would tell you to MSM.

*** Know how to use the MSMTool, Don't start before figuring it out. https://xdaforums.com/t/op9pro-repository-of-msm-unbrick-tools-tmo-glo-in.4272549/

*** DO NOT ATTEMPT THIS IF YOU ARE NOT FAMILIAR WITH ANY STEP DESCRIBED IN THIS GUIDE

General notes:

NR and LTE bands are read only and not modifiable, not in modem.img or any other partition. You cannot get global bands by flashing a global ROM, nor can you lose TMO bands by flashing another ROM.

If you wish to unlock the bootloader without unlock_code.bin, you MUST do it from OOS11. If you upgrade to OOS12/OOS13, you will need the unlock_code.

The T-Mobile NON-HLOS is no longer necessary as of OOS13 release. If you must stay on OOS11 or OOS12, make a backup of the T-Mobile modem partition for the OOS version you want to stay on.

Dual-SIM does not work.

Prerequisites:

This conversion will not SIM unlock you. You can only SIM unlock on a T-Mobile ROM, meaning if you want to do that in the future, you must go back to a TMO ROM (and wipe)

Familiarity with MSMDownload tools. If the regular MSM tool doesn't work for you, turn back now.

Back up all data, it will be wiped.

Download:

Conversion: AndroidFileHost (MD5: 8cc3e5e81d15e28bb10dc5f1e7bd4133)

Steps:

Run the MSM file normally.

Unlock your bootloader if necessary. Now is the time if you don't have unlock.bin

Patch up to Android 13 while on Wifi if your mobile data doesn't work. Use this time to go to another variant (GLO, IN, etc) as needed.

Fastboot method:

* The steps in this guide will convert your TMO LE2127 to EU 11.2.3.3. This is intentionally not the latest version to force you to take a FULL OTA.

* You must already have the bootloader unlocked with an unlock_code.bin

* You can use this guide to convert to your own variant by extracting payload and flashing from a full OTA. No support will be given if you choose to manually do it.

* Use the latest platform tools: https://developer.android.com/studio/releases/platform-tools. DO NOT USE MINIMAL ADB, it has not been updated for years.

* Fastboot can't see your device? https://xdaforums.com/t/guide-fix-d...stboot-mode-windows-10.4194491/#post-84943553

* Test that you can boot to fastbootd prior to starting, "fastboot reboot fastboot" and you should see the OnePlus logo with the text "fastbootd" on top. Do not continue if this doesn't work.

* When encountering issues, describe all steps you took, paste any outputs, and take screenshots. Make sure to include the output of fastboot --version in your post.

Steps to convert your device to EU

You need your device bootloader and SIM unlocked. See https://xdaforums.com/t/how-to-unlock-the-t-mobile-bootloader.4256319/

Download and unzip the conversion package:

EU 11.2.3.3

MD5: cb949816fa80d5f0d984359b819426c3

Boot to bootloader.

From adb:

adb reboot bootloader

From fastboot/bootloader:

fastboot reboot bootloader

Execute "flash_all.bat" from the unzipped directory from step 2.

Reboot into recovery, and wipe everything.

Once fully booted, connect to WiFi and download Oxygen Updater and take the FULL OTA (11.2.4.4 as of this writing)

Do a regular reboot, don't reboot to fastbootd, bootloader, recovery, etc. Reboot to system.

Reboot into bootloader or fastbootd, and flash the T-Mobile modem.

fastboot flash modem NON-HLOS.bin

After each Full OTA you take in the future, you will need to repeat step 7 & 8

FAQ

Q: The most recent version is 11.2.4.4, why is this guide converting to 11.2.3.3?

A: You didn't read the full post.

Q: Why is there no conversion to Global?

A: Global variant uses the Google incremental OTA server, and full OTAs are not always provided. There's no functional difference, and EU has better privacy requirements anyways. You can do it manually by extracting payload.bin and running the same flash script. No support will be provided if you choose to flash to Global.

Q: Can I re-lock the bootloader after conversion?

A: While you can go ahead lock your bootloader after conversion, I don't recommend it as you will not be able to take incremental OTA due to modem changes, and lose connectivity if you take a full OTA. To fix it, you need to unlock the bootloader and end up wiping.

Q: Where is the Linux guide?

A: Too lazy, if you're using Linux, you can make your own bash script from the Windows batch script.

Q: Help, I see is 'fastboot' is not recognized as an internal or external command.

A: Your fastboot.exe is not in your $PATH. You can either drop fastboot into the same folder as flash-all.bat, or you can fix your $PATH

Q: Help, I cannot boot to fastbootd because previous flashes messed things up.

A: MSM

Q: Help, "not enough space to resize [system, product, system_ext]"

A: MSM. If the error is on Vendor, it might still work and boot with a generic Android boot logo. If so, take the full OTA as described in the guide.

Q: Help, something went wrong!

A: Post in here the starting state of your device (Device model, ROM version, etc), all steps you've taken since unlocking bootloader, and the output of whatever is causing you trouble. Chances are, the answer is MSM, but read my MSM warning.

Q: Help, I see "Invalid sparse file format at header magic"

A: Ignore this message, not an error

Fixing the About Phone page

These steps are optional, and is only applicable for OOS11. It will fix the "About Phone" page. There are no performance differences. All that's done is change build.prop to reflect the correct model.

You need to rooted with Magisk

Download and install the magisk module: here

Reboot, the About Phone page is filled out.

Instructions to Bootloader Unlock a SIM locked device:

In ADB:

adb shell pm uninstall -k --user 0 com.qualcomm.qti.uim

Reboot

Go to dev settings and enable OEM Unlock

Reboot to bootloader

fastboot oem unlock

No unlock.bin needed

Issues so far (please post if you run into more):

If your mobile data doesn't work on OOS13, PM me details including HW Revision in About Phone.

What appears to work:

Everything else.

Much appreciation for everyone that help develop and test

@razercortex

@retryfail

@amaroney55