If your little one was on live before being in your care, then I recommend sticking with live for 3 meals (without a refusal). This way, you can ensure that they're eating properly in your care and not worrying about your new one missing meals because of unfamiliar food. After the 3 straight meals is when I switch mine over to f/t.
Edit: If they were on frozen before, then stick with f/t. It's all about consistency and being comfortable when you first bring a new little one home.