Once you purchase Lightroom CC, you would need to upgrade the Lightroom 5 catalog to the newer version.
Please check the link for how to upgrade the catalog to the newer version and how it works:Photoshop Lightroom catalog FAQ
Also if you want you may uninstall Lightroom 5 from your computer.
When you first run Lightroom CC 2015, the LR 5 catalog will be automatically upgraded to a LR CC 2015 catalog (well, almost automatically, you will get a dialog box giving you the choice to upgrade and you have to click on "Upgrade").
It is optional to delete LR 5 at that point.