If a siding installation contractor is considering installing vinyl siding over existing wood siding, they will check to see that the wood siding is in great shape. The wood cannot show any signs of rot, mildew or mold. Also, the wood siding must be securely fastened to the wall sheathing or studs. If there are bad boards, they must be replaced. All door and window trim must be removed. A qualified siding installation pro will be able to remove existing siding before installing the new siding on your home. If your home has wood siding that is in great shape, the expert may be able to install the new siding over the old.