Yes of course.

Models.com members can downgrade and upgrade at anytime and as often as they want. Downgrades are effective on the next billing renewal cycle.

You can downgrade and upgrade your membership via your Account Settings > Payment Settings

Downgrading your membership from Pro to Free will not result in any saved info being lost. Lists you had created that surpass the limits of a free account will not be deleted once the membership is downgraded, however you may lose access to editing/creating more lists.