POST#1 POST#2 You will have to disassemble both handles. I don't have one in front of me now, so I will have to do this from memory. At the bottom of the shaft there is a spring and retainer. This is removed by removing the pin that holds them on the shaft. Once removed, you will separate the pieces, setting them aside to be reused later. The new handle shaft will be longer and will have to be shortened and a hole drilled to accept the pin. The bezel and shaft housing from the barn door handle should be cleaned up for reuse. Assemble the new door handle with the barn door piece (less the handle) and install on door.
Somewhere, I think I have seen pictures from Achipmunk, hopefully he will check in and elaborate on what he did to accomplish this task.