I really would not go for a gerbil if we know the snake takes mice just fine.

If you have a reason you want to make the switch to rats, she will probably come around with some "tough love". You can make the transition easier by scenting the rats that you offer to her with mouse smell, and putting some dirty mouse litter in or near her cage at feeding time.

However, there is no need to switch her from mice to rats. She can do just fine for her whole life on mice. Just give her enough mice to make an appropriate sized meal.