[OnePlus 9] centifanto:
Howdy.

So, not trying to steal anyone's thunder, but this Convert T-Mobile OnePlus 9 to Global (or other) firmware thread has not been updated by the OP (@Lomeli12) in awhile, and I ran into some hiccups when following the directions, along with a couple clarifications scattered across different threads. Credit of course to the original thread above that is still very helpful minus being out of date, @craznazn for the conversion script, @Ju5t3nC4s3 for the OEM toggling tool, and the advice of other kind XDAers responding to my questions.

The key here, that the thread above does not indicate, is that the conversion takes you back to 11.2.2, and that when you try the OEM updater or the Oxygen Updater, both just want to take you straight to Android 12 (C.48). For whatever reason, probably some preliminary prep that takes place in later Android 11 updates, the C.48 update fails exactly at 25%. I tried the conversion and update multiple times, just to make sure that wasn't a corruption issue.

After this dead end, I decided to check out the sadly not current official OTA download site to see what was available. Interestingly, their current OTA was not C.48, it was 11.2.10.10. I quickly downloaded that, and lo and behold the update succeeded! Afterwards I checked and my device model had changed to the correct LE2115 as I wanted it too (it had not changed after running the conversion script).

Then I checked the system updater, and C.48 was waiting. Installed, rebooted, and boom I was on Android 12. Then C.63 with the August 5th security update was ready. Installed seamlessly.

Helpful tips/findings/clarifications:

You will receive this final error at the end of the conversion script, but you can ignore

fastboot -w
Erasing 'userdata' OKAY [ 0.317s]
CreateProcess failed: The system cannot find the file specified. (2)
fastboot: error: Cannot generate image for userdata
Done!
Please reboot to recovery via the menus and wipe everything prior to boot

Your device model will still show TMo (2117) until you successfully apply the 11.2.10.10 update

For anyone confused by multiple threads that are seemingly contradictory, you do NOT have to be SIM unlocked to do this. You just need this nifty little tool from @Ju5t3nC4s3 that enables you to toggle the OEM unlock and subsequently get the unlock code to submit to 1+ and then unlock the bootloader with the token 1+ sends you

If you don't plan on leaving TMo, then leaving SIM locked is fine. If you do decide to change carriers, you'll need this MSM to flash back to TMo firmware to then request SIM unlock

You definitely can apply OTAs with an unlocked bootloader. Just not a rooted boot.

If you have root, you can easily pull your modem and persist imgs via Partitions Backup & Restore

If you do not have root, I received confirmation from @EtherealRemnant in this thread that the modem and persist that I pulled via the above mentioned MSM are in fact valid.

Root is easy as booting, NOT flashing, a patched boot, then using Magisk manager to patch. This enables your to easily revert back to stock boot. If you flash, your stock boot is gone

Hope this helps someone else avoid many hours of thread scanning :)