This would be a good feature request that you could send to support@code.org. Right now, I believe the only workaround would be to create a separate section for advanced students and have them join that section where you could open things up sooner and/or leave all units open. I believe that they would need to be removed from your first section because if they are in multiple sections of the same course, any hidden units will still be hidden.
Not an elegant solution, but it should work. Their account should still be the same, so moving them to another section should not remove any work they have already done.
Someone please correct me if I’m wrong.
Mike