The first time you launch Lightroom CC, it will detect a previous install of Lightroom and will upgrade a COPY of your old catalog to the latest version. The original catalog will be left intact in its original location.
In fact, the entire old version of Lightroom can be left intact until you're confident that the new version is working to your satisfaction. Just keep in mind that the two versions will not be able to share a common catalog, so you can't bounce back and forth between them. LR CC makes a copy of the working catalog and upgrades it, and the newly upgraded version is not readable by the older version. So the two will remain separate but both working.
I hope that helps!