How do I switch to a new SIM card?

The title says it all: How do I switch to a new SIM card?

I bought a new iPhone 6, which has a nano SIM slot (and an Apple SIM already installed), so I can't simply swap out the SIM from my Nexus 5.

A quick search pointed me to http://t-mobile.com/SIM or http://tmobile.com/simupgrade, but these sites allow me to log in, but then take me either to the T-Mobile home page or to https://manage.t-mobile.com page with no option to change SIMs on either page. The support docs still point to http://t-mobile.com/SIM but that clearly doesn't work.

If it makes a difference, I'm on a pre-paid plan.

Is my only option either calling or going to a T-Mobile store?