[OnePlus 9 Pro] Appreciative:
Modemst1/2 do have unique information. Modem_a can be shared. Modem_a/b is different from modemst1/2 and modem_a/b is the one in question for this device.

However, you should back up modemst1/2 and efs/persist anyway because then you really have nothing to fear if something goes wrong.

As of right now, while in oos11, it's already possible to unlock the bootloader of the le2127 without contacting OnePlus or t-mobile. By converting to EU, while on oos11, you can bypass the OnePlus+Tmobile bootloader token requirement and unlock the bootloader.

It sounds like they are already on global anyway.

What I did may give you peace of mind also:

Rebooted my device into edl mode with msm tool open and took backups of partitions I wanted (modemst1/2 efs/persist and boot)

Used One+TooL to make OEM Unlock toggle available in developer settings. (This was needed because I was not sim unlocked at the time. The tool or adb can be used to bypass that and make the toggle visible)

Got my bootloader token in fastboot and submitted it to OnePlus website. I know it takes a week (I've since heard it is faster using support chat or email and the automatic tool is providing wrong unlock bins).

Knowing I will be waiting a week, I did the Tmo->EU conversion, unlocked my bootloader, rooted using the boot.img I made a backup of, flashed my modem_a (not modemst1/2) back in and set everything up. Staying on oos11, this is no problem. Updating to oos12/13 after the conversion, you will need the unlock token again. As I understand it, the unlock token bypass is only present in oos11 so it's advised to do it then.

This is the furthest you'd need to go most likely, you can go from Eu to global. The connectivity issues I've read, seem to come for those who upgrade Eu to the latest versions of oos13. They solve it by going to Global instead.

(A week later I received my unlock.bin from OnePlus. I used the regular Tmo msm tool to go back to stock Tmo. Verified my bootloader unlock token worked in stock form and it did. I unlocked it, rooted again, and set up https monitoring to see the exchange happening during sim unlock. Then I requested my sim unlock and eventually found someone not dumb to help me out. I logged the request I was interested in and went back to Eu again, rooted/unlocked bootloader again, flashed my modem_a again and have been here ever since)

I can't speak for custom roms or oos13 but I can say for sure that on OOS11, it is possible to bypass the sim unlock requirement for bootloader, possible to unlock the bootloader without the token, it is possible to have unlocked bootloader, root/magisk and retain L1 DRM for netflix/disney+, google pay works, android auto works and I'm showing up as device certified in play store. So long as you install the appropriate magisk modules of course.

The LE2127 is a lil bit different than the others but it's definitely not a nightmare or very risky if you take precautions. Take backups of your partitions, keep the msm tools handy and you'll be just fine. Every one of us who have done any flashing on xda, have had to redo or start over at some point on some device. Luckily, there are a lot of us in this section that have the le2127 to help ease your mind and assure you that it's gonna be ok. We have been blessed by a few guys who did the hard work before us and figured out the bypasses and fixes to various issues.

Since this phone is currently the last OnePlus device to have real MSM tool and custom rom development, it's looking real good to be a 9P owner. This phone has a lot of good hardware, specs, options and support. I think you've done good to research beforehand. I think you'll be happy with the device when you get it set up how you want it also.

For taking backups in the msm tool, you put your phone into edl mode and connect it with msm tool open, press F8. That will bring up a list of partitions you can backup. Select the ones you want (especially modemst1 and persist), type the password "oneplus" and press readback. It will save the partitions to C:\. I suggest making a backup of the backups but I'm extra cautious. Don't sweat it tho, you'll be fine with le2127