My mom cancelled our family plan and switched to a new carrier. If I need to do a factory reset on my old phone, can I still use it as a music player? Or will it try to reactivate the carrier account?
You can use old phones on Wi-Fi without an issue. It will attempt to search for a network and activate, but it will fail because the SIM no longer belongs to an active account. It can't reactivate the old family plan on the previous carrier.
I would enable air plane mode to disable the cell radios to conserve battery, it will disable Wi-Fi and Bluetooth, but you can toggle those back on while still in airplane mode.