Switching can be done at anytime, usually the younger they are switch the easier it is.
Some breeders switch their animals some do not, when you produce 100, 200. 500 animals feeding is time consuming and switching is not always an option.
As a breeder I switch animals on demand for customers but only guarantee that an animal is switch if the animal takes at least 5 consecutive F/T meals.
If you know how to do it switching can be easy however you can have have a stubborn animal that may not switch right away and take several months to switch.
If you want an animal that eats F/T ask the breeder if the animal eats F/T and if so how many F/T meals the animal has taken (eating 1 f/t prey does not qualify has being switch, nor does an animal going back and forth.)