[Samsung Galaxy Note 10+ Questions & Answers] TheXev:
I have moved a Note 10+ from it's original T-Mobile firmware to Verizon and then back, so hopefully, I can explain this process easier then that original bloody guide (which I pulled my own hair out over). To perform the operation ONE WAY, you must have an activated SIM card on the target carrier.

First thing you want to do is perform a full backup for all of your files, as they may get wiped during this process (and more than likely will). I used Samsung's backup to an SDCard I inserted into mine. Remove the SIM Card from the phone before starting this process. You may also wish to remove your Samsung account and any security codes/methods you have when flashing. That might be too much, but I always do it just encase things go wrong for some reason.

Obtain the newest firmwares using SamFirm (currently 0.5.0). If you cannot find a Region in the drop-down listing, just type the three letter code manually into the region box to search for it (this is how I obtained the USC firmware for the SM-N975U1). You want the universal SM-N975U1 Firmware (USC), and the target SM-N975U T-Mobile firmware or whatever other carrier you want to target(yes, U only).

At the time of me typing this, the SM-N975U1 USC firmware is N975U1UES4CTF2 dated 07/06/2020, and the SM-N975U TMB firmware is newer, being N975USQS4CTF3 07/27/2020. It is fine that the TMobile firmware is newer, just realize that flashing FORWARD to newer firmware works, but flashing firmware BACKWARDS is impossible. sammobile[DOT]com has a list of firmwares with PDA and CSC, so you can confirm there what firmware codes are newer then others. (I don't recommend downloading roms from them, unless you feel like shelling out money! I did have to use this site to get the non-U VZW firmware) This, in particular, makes it a pain to flash TO Verizon, as their firmware is typically a month behind everyone else. Ideally, you want the baseband versions of all of the ROM's to match perfectly, but this can be difficult to do as they are always changing.

The last time I performed this, which was only a few months ago before selling one of my Note 10 Plus's, I used Odin3.13.3 3B PatcheD.

Use Oden to flash the U1 USC firmware. Start the phone up with airplane mode off and no sim card. Insert the SimCard and you should see a popup that says "reboot to apply new carrier settings." Reboot the phone and ensure that things are working okay at that point. If you did not see this message, shut down the phone with no sim card and attempt again until you see the message. If you do not see this message, the carrier settings will not apply and you will not be able to flash the target SM-N975U firmware. You can verify this worked correctly by navigating to Settings=>Software Information=>Service provider SW ver. You should see something like "VZW/VZW/TMB" in your particular case. The first two will always be the original hardware's carrier, and the last one should be your target carrier to proceed.

After ensuring you have the correct carrier settings, flash the target firmware of your choice. Using Odin. It is normally important to flash the HOME CSC to get all of the carrier specific stuff, this may include wifi calling settings and software, but I am unsure (this is the main reason you backup the phone). This is the step that will clean wipe your phone in my experience.

To anyone later reading this, from the research I did when I first did this months ago... note that switching the carrier firmware may not make all of that carriers features work. I remember reading that people still could not get AT&T Wifi calling to work, even after contacting AT&T and getting the phone put on the wifi calling whitelist. I don't think Verizon and T-Mobile have those particular issues.

If you ever want to flash the phone back to it's original firmware for any reason, you MUST obtain a SIMCard from that carrier, otherwise you will be stuck!