The switching over to the creative cloud is very easy to do. You sign up and then install the Creative Cloud Application Manager. That manager will enable you to install Lightroom, Photoshop and Bridge. After you install the programs, when you start Lightroom it will recognize your catalog that you have been using with Lightroom 3. You will be prompted to allow Lightroom CC to upgrade the catalog. This will create a new copy of the catalog, leaving the old one intact and usable with Lightroom 3. After Lightroom CC converts the catalog, all the work that you have done previously will still be there, and will include all of the adjustments that you have done in the older version. You can keep Lightroom 3 on your computer for as long as you wish. The programs are completely independent from each other. When you decide that you no longer need Lightroom 3 it is safe to uninstall it.
You just have to download Lightroom CC and on first launch, Lightroom would automatically ask you to upgrade your catalog.
Once you will upgrade the catalog, it will restore all your edits and presets.
This will keep your Lightroom 3 Catalog intact and will create Lightroom CC Catalog with all information upgraded.