Yes, as zinekeller said, they (T-Mobile support presumably) simply changed what SIM card was assigned to my account. Luckily, I received a disconnection notice

Yes, as zinekeller said, they (T-Mobile support presumably) simply changed what SIM card was assigned to my account. Luckily, I received a disconnection notice on my phone as soon as this happened and was able to call T-Mobile (with my computer and Google Voice) to get it changed back.