[OnePlus 7T Guides, News, & Discussion] urbanman2004:
If you purchased a OnePlus 7T which went on sale recently, but still aren't able to replace the T-Mobile firmware with the internat'l variant then you're in luck. I came across a few guides which were a smoking gun, but I was never fully able to get it to work successfully and "failed miserably". And a few posts I read helped spur some ideas and inspired me to think outside the box in order to come up with a much more predictable viable solution here. Throughout this guide, I have linked the page or post based on information I gathered to put this guide together in order to credit that person instead of having to name drop them individually. The instructions given were made with the expectation that you already have the bootloader unlocked and installed the necessary drivers with adb setup. I followed this guide here as point of reference which I think could be potentially bypassed by using this method instead to completely streamline the process. Best of all, I spent 15 hours yesterday trying to figure this out so hopefully you won't have to go through what I did. NOW LET'S BEGIN!!!

FLASH OPERATIONS

I. Just to give a little background for those of you who followed the guide I referenced, whenever I attempted to flash the 10.0.13-GLOBAL fastboot rom it would display the error message "Flashing is not allowed for Critical Partitions" multiple times then auto reboot T-Mobile logo splash screen, indicating that the process failed.

At this point, hold Vol Up + Power till the device shuts down and boot into fastboot.

On the PC, download the All In One v2.0.1.3 tool.

Download 10.0.3-HD65AA-OnePlus7TOxygen_14.I.05_OTA_005_all_1909252229_1be2cbaa2-FASTBOOT.zip or from here.

Open All In One v2.0.1.3 –> Flash Factory Images –> Browse directory for the fastboot rom –> check off "Wipe all data" and "Reboot after flash" –> Flash Rom.

During flash process, it will reboot once, NOT into fastboot, but stock recovery menu (FastbootD), and pick up where it left off and resume flashing the rest in which the process can be visible from the PC.

After completion, the device will be updated to Stock OTA

10.0.3HD65AA

rom image.

II. Download and extract "10.0.13-GLOBAL-OnePlus7TOxygen_14.O.19_OTA_019_all_2009281709_1026-FASTBOOT.zip" into a folder called "adb" in the root of the C: drive.

III. While the phone is turned on, we must boot into FastbootD. The fastbootD screen has the OnePlus logo centered at the top with 3 language choices. Run "cmd" as admin and type:

adb reboot bootloader
fastboot reboot fastboot

IV. Enter this command to check what version of Fastboot you've booted into:

fastboot getvar is-userspace

"Yes" means you're in FastbootD.

V. To flash the "10.0.13-GLOBAL" fastboot rom, run the bat script by entering these commands:

cd C:/adb
flash-all.bat

During flash process, it will reboot once into FastbootD again (stock recovery menu) and pick up where it left off and resume flashing the rest in which the process can be visible from the PC.

After completion, the device will be updated to Stock OTA

10.0.13HD65AA

rom image.

Take the opportunity to perform a system update to Oxygen OS 10.0.15.HD65AA.

Enable developer mode: Go to Settings –> About phone (at bottom) –> tap Build number (towards right) until it says Developer Mode is enabled.

Go to Developer Options –> Enable USB Debugging and "allow". Go to "Default USB configuration" and change it to "File Transfer".

ROOT THE DEVICE

VI. Download and copy magisk_patched 10.0.8 international (global) into the ADB folder. Rename it to "modded-boot.img". Flash the modded-boot.img file to achieve temporary root. While the phone is turned on, in cmd type:

adb reboot bootloader
cd C:/adb
fastboot boot modded-boot.img

The device will reboot on its own once the process has completed.

VII. Download Magisk Manager and copy it to the phone's internal storage and use the native "Files" app to install the apk.

Launch Magisk Manager. Click "Cancel" on the "Requires Additional Setup" message.

Update "Manager" if using an outdated apk. Update "Magisk" via Direct Install and Reboot.

After the reboot, the device will retain root which can be verified using apps like Root Checker in the Play Store.

Someone who hasn't already tried converting their stock T-Mobile firmware to the OnePlus Internat'l firmware variant and is interested enough to follow these steps, please update me with your results after going through the process to confirm it was successful, I would greatly appreciate it.

If you have any helpful suggestions that could offer more insight or if there seems to be any holes or missing explanations within my instructions missing feel free to comment.

Kudos to @mauronofrio, without his tools none of this would be possible