Reverting back from a forest/domain functional level upgrade would require a full forest or domain restore, which Recovery Manager for AD Forest Edition can perform, provided the Forest Recovery project has been configured prior to the upgrade and that the proper backups exist.
Please note, this would not be a trivial matter and it is required that you contact Microsoft first and have Microsoft confirm that a full domain for forest restore is the only option.
Configuration of the Forest Recovery project is outside of the scope of Support. If you require assistance with configuring the project, please contact your Sales agent to have them engage our Professional Services department.