What is an EB-2 to EB-3 downgrade?

How do you file an EB-2 to EB-3 downgrade application?

Can an employee legally pay for the costs of an I-140 petition to downgrade from EB-2 to EB-3?

Can an employee legally pay for the costs of an I-485 Adjustment of Status application filed concurrently with a downgrade petition?