If your child is already trained, she has probably used a potty rather than a toilet. Now, in preparation for preschool, she needs to feel comfortable using the toilet both at home and elsewhere. If she hasn't mastered the skill already, she should also start to stay dry at night. It makes it easier, too, if she can pull her clothes up and down by herself. Physical and mental readiness aren't the only factors involved.
Motivation is key.
If your child demonstrates a desire for independence and shows an interest in imitating others' bathroom habits, it may be an auspicious time to plunge into the process. Children may figure out how to have a bowel movement before they learn to urinate in the potty because they typically only have BMs once or twice a day, usually at the same time. Take her shopping with you, and let her help pick it out. She'll be much more excited about using it. After she's happily made this transition, you can put her potty away.