Hello @cassbruni,
When you get the new phone it should come with a pre-activated SIM card that you just have to update on your account and put it in your new phone and the old one will be deactivated. To learn how to update the SIM card see here.
Alternatively, if your current SIM card is the same size as what the new phone requires and if it's an LTE SIM you can just put the old one in the new phone and it will work. No need to activate the new SIM card just keep it safe in case you need it in the future.
