5G and future 6G networks support diverse combinations of access technologies, architectures, and radio frequencies, with each combination termed as a “band” henceforth. Through comprehensive measurements in 12 cities across 5 countries, we experimentally show that operator-configured default bands are often highly suboptimal, particularly under mobility. We then propose smart band switching, where a UE’s band can be dynamically changed to improve the network performance and boost the application QoE. We discuss challenges, opportunities, and design choices for building a practical smart band switching system. We further develop preliminary UE-side band-switching logic on commodity smartphones, and evaluate it on commercial 5G networks.