I'm having the same problem. I purchased a license to Lightroom several years ago, and the upgraded to CC. I've cancelled by CC subscription with the understanding that I could go back to using my old license, but unfortunately it wants my license number. Does anyone know where I can find it?
If you have the same computer you installed LR 4 or 5 on you can find the serial number in a text file on the hard drive.
See this article for the exact location of that file.