On the same Family Mobile SIM I got before the switch to Verizon

So I’ve been transferring the same SIM card to every new phone I’ve gotten for like a decade, and I’m pretty sure it still uses the T-Mobile network. I’ve had issues with dead zones for years and was excited to get on better coverage but when I tried to transfer to a new SIM years ago I ran into some problem I can’t remember and gave up. Does anyone have experience transferring their number to a new SIM on family mobile so I can know what to look out for?