I've had somewhere around 80-90 ball pythons over the last few years and consider myself very good at getting them feeding f/t rats. I've successfully switched many from mice to rats as well. Almost every one I've purchased has been on live before I got them and all my hatchlings start on live hoppers. I've never failed at getting a young snake switched to f/t, some took much more time and effort but they've all switched. I've had one that never switched from mice but he's taken f/t. I currently have 2 young females that are being difficult and only want live rats but I still have hope for them.
I've had one female that I got as a sub adult and she never switched to f/t after years of trying she had taken one f/t and that was it. Months at a time refusing which would lead to weight loss and I'd be forced to give her anything she would accept to get weight back up and then start over trying to switch without luck. A couple years of this and I finally gave up. She regularly refused live food as well so she was just a terrible eater to begin with and just never had a good enough feeding response to get her switched. So I would have to say that some will never be able to be switched to f/t but it is an extreme rarity.