That Sprint to T-Mobile conversion – not quite seamless, but not a pain (updated)
If you’re on Sprint and not aware, you’re (possibly) going to need to get a new SIM eventually as the Sprint Network is going away. I’m not entirely clear on why the SIM replacement needs to happen as I thought that just stored an identity numbers and pass key, but we need new ones for some reason.
TL;DR – you might need to reinstall Facebook, you will lose voicemail.

I happened to be at a kid’s event today next to a T-Mobile store and took our phones in. The email I received said it was going to be a pretty simple swap, reboot, and gone. Close, but not quite. This was on my Galaxy Note 8 and the wife’s Galaxy S9+.
I was informed by the T-Mobile sales associate that the main issue was that T-Mobile was on a different frequency than Sprint which is why we needed a new SIM… I didn’t really want to argue that a subscriber ID number could be programmed in, and many phones have E-SIMs, and the phone handles the frequency… for some reason they’re swapping SIMs and I needed that to access some of T-Mobile… whatever.
What was not mentioned is your voicemail is going away, so yeah take care of any of that Sprint voicemail. It’ll need to be re-set up. I lost a voicemail from someone. Also give it a good 10 minutes from when you switch to T-Mobile before it works, don’t be like me who tests everything 2 seconds after getting back to a rah rah Princess event and realizes the voicemail doesn’t work (it did shortly).
Also be aware that everything installed via the Sprint service loader that your phone was activated on gets removed. For my wife, we’ve not found anything on her S10 that was missing other than some Sprint spamware. For me it uninstalled Facebook, Facebook Messenger, and a couple of Sprint apps that didn’t particularly matter. The Facebook missing was a bit unexpected as today is the day after all the iPhone users were suddenly logged out of FB… but it was unrelated. As Facebook installs Facebook Messenger my guess is that’s why that app disappeared. Just a chain.
Amazon shopping seems to have been a victim of T-Mobile’s Sprint toss as well… reinstalling didn’t even ask for a password for my account though.
Call forwarding also didn’t transfer, not that that’s a huge deal. Voicemail setup was pretty easy. Had to update the T-Mobile software after it showed as they seem to push an older version. Going into Google’s Android Messages it asked me to agree to use enhanced messaging again but all my old messages were there.
Not a terrible Sprint transition.
Updated: T-Mobile tower speed tests are slower, but not much (could be unrelated / weather,) and visual voicemail is… meh. Calls seem about the same. Voicemail recording seems a bit better (or people have been leaving voicemails more professionally).
While I am on T-Mobile now it still redirects me to the Sprint billing side.