https://forum.xda-developers.com/t/guide-convert-tmo-to-global-eu-or-other-variant.4188491/page-35

And what do you mean by converting it to international?

You can convert it to the international to get faster update I have the T-Mobile version I have the always on display if came with the October update and I just got an update this morning to the November

I'm pretty sure you can get TMO to unlock it for you after it has been active 40 days or more. You're still on the hook to pay for it, but some people want to swap SIM cards for international travel.

There are some advantages when buying from TMO. For one thing if something goes wrong with the phone, you can go to the carrier and get an instant phone replacement. The same is not true when buying through OnePlus directly.

There are horror stories well documented in Reddit about how awful OnePlus support is. When you mail your device in for service, it may be a while before you will see it back. When you do get it back, there is a risk that the repairs may not be of good quality.

The other thing about updates is that it is important to differentiate between major OS releases and the regular security updates that come every other month.

It is true that carrier variants get the OS releases a little later. This is not a big deal go me.

My TMO 7t does get regular updates every other month with no problems.

In the end, what you need to decide is how important those major OS releases are to you.

Also when you finish paying off the phone, you  can ask the carrier to unlock it for you.

You are correct.  I bought my 8T in store and they automatically threw away my old SIM, grabbed the new one out of the box, and said "you need to use this SIM card now". hahaha.  I did not argue.

Can confirm. No case or screen protector with the TMO version. You DO get a new TMO SIM card, though. I've read elsewhere that it is best to use that one and do a SIM switch on-line. Something to do with the new SIM being better for the 5G provisioning.

The T-Mobile OnePlus 8T is one of those rare cases where the phone actually seems BETTER than the unlocked variant.  I did a ton of research before buying mine (T-Mobile version).  You are getting full IP68 rating.  You are getting the official OnePlus messages and phone app preloaded on the phone (this might not be the biggest deal, but some people REALLY want the OnePlus apps not the Google apps with no good way to get them on the 8T).  T-Mobile version is also optimized to use both their 600MHz 5G network as well as their newer 2.5GHz 5G network.  You also getting one more step for software updates to get ironed out/approved.  This is sometimes overlooked, but (from the sounds of it) the new unlocked version is a complete dumpster fire.  It has not passed T-Mobile certification yet.  I am sure by the time it does, the kinks will be ironed out.  

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You are getting not very much in the way of T-Mobile bloat (3 or 4 T-Mobile apps, the opening "T-Mobile" splash screen after res

I am getting the T-Mobile variant of the 8T because they are giving it away for free when you switch over to them and open a new line. I hate to get a carrier variant, but it's free so why not. At least the T-Mobile version has an IP rating.

What are the differences between the unlocked and T-Mobile versions? So far, I only know that the T-Mobile version has an IP rating, locked bootloader, some features like always on display removed, and potentially delayed updates.

T-Mobile Oneplus 8T vs Unlocked