Yes, swapping in a both GSM/CDMA compatible phone is likely the best way to ensure a smooth transition to T-mobile/Tello.
"If you intend to buy a new phone in the near future, we recommend that you ensure that it is both GSM and CDMA compatible and that it was released no earlier than 2018. To best use the new network, your phone needs to have, as minimum requirements LTE frequency bands 2, 4, 12 + VoLTE and when talking about an ideal connection: bands 2, 4, 12, 66, 71 + VoLTE. Key takeaway: Regardless of the phone brand you fancy, always go for a device that supports voice over 4G (VoLTE), which will help make the most of the newly expanded network."
blog.tello.com/news/tello-and-the-telecom-giants-merger/