I've had ball pythons that have starved themselves to death, but they were babies that had never started eating in the first place. Healthy ball pythons with good weight can go a very VERY long time without eating. Is there any particular reason that you want to switch it's food? I don't remember ever having a problem with a rat eater taking mice, my problems have always been the other way around with mouse eaters refusing rats. In my experience it's always been easier for me to get them to eat mice then to eat rats.