Process to change phones, from iPhone 6S to iPhone 7

Anyone been through the process of switching phones using Tello + T-Mobile SIM, from an iPhone 6S using T-Mobile SIM, to an iPhone 7?

I see that, after logging into my Tello account, I can go to My Phone \ Want to replace your phone? \ Bring a new phone, and enter the replacement phone's IMEI/ESN, then click CHANGE DEVICE.

Is that all I have to do? Need a new SIM? Any caveats I should be aware?

You should be contacted by Tello regarding migration to T-Mobile. They will send you a T-Mobile SIM for free. Once you receive it and activate it from your account, you can use your SIM in any unlocked GSM phone, no need to change IMEI in your account. T-Mobile SIM is not locked to specific IMEI.