I have been trying to deregister Lightroom 5 on an existing computer to then re register on its replacement. Although some of the literature claims I can load a single user application on two computers it does not work for me. When I use the product

I have been trying to deregister Lightroom 5 on an existing computer to then re register on its replacement. Although some of the literature claims I can load a single user application on two computers it does not work for me. When I use the product serial on the new machine I get a message code already in use and no further options. How do I de register