From Lightroom 6 to Lightroom CC

First of all, there is no Lightroom CC. That name went away quite a while ago when Adobe decided to confuse everyone. There's Lightroom Classic, which is the familiar desktop version which uses a local catalog and stores images on your computer. And there is Lightroom, which stores everything in the cloud and enables you to share images on all your devices. I assume you are considering switching to Lightroom Classic.

Lightroom Classic, when you first run it, will prompt you to upgrade your previous catalog. This is necessary to make your catalog compatible with Lightroom Classic. The upgrade process will create an entirely new copy, leaving the old catalog intact on your computer. The new catalog will have the same name except it will have a -2 appended to its name. It will have everything included in it that was in the old catalog, including all of the work you have done and all of your collections. Be aware, however, that it will NOT be backward compatible with Lightroom 6. So if you ever decide to go back to Lightroom 6 any work that you have done in that catalog will not be accessible. The images will be on your hard drive, and you can import them using Lightroom 6 and edit them again.

Hopefully, you will have an installer for Lightroom 6.14 should you decide to return to it because it isn't available anywhere anymore. You have to contact Adobe personally and directly in order to obtain the installer. Unless you have no appreciation for the improvements that have been made since Lightroom 6, I see no reason why you would ever want to return to it. It would definitely be a step backward in my opinion.