Can we change full-timer to part-time after maternity leave?

Q. We are a small company and have seven employees. One of our employees recently went out on a leave of absence for pregnancy. During that time, we hired a replacement worker to do the same job. The replacement only worked part time, but was still able to complete work that our employee did full time. When our employee returns to work, we would like to change her job status from full time to part time. Is this legal?