A steam diverter apparatus for forming part of a downhole tubing string is provided. The apparatus may be used in a SAGD operation to control flow of steam into a formation. The apparatus includes a tubular housing defining steam-admitting ports, and containing a moveable shift sleeve. The apparatus is modifiable for use with position shifting tools of different outer diameters by interchanging first and second sets of the shift sleeve and insert rings having different minimum inner diameters within the housing. The open flow area of the apparatus is modifiable by plugs that block the ports and are removably attached to the housing. The plug, the shift sleeve, or both of them may be made of a dissolvable material, so that they can be dissolved by a chemical solution injected into the tubing string, for the purpose of controlling steam flow through the ports.