When you first run Lightroom CC 2015, it should find your existing Lightroom 5.7 catalog, copy it and upgrade the copy for use in Lightroom CC 2015. All you have to do is click on YES or OK or whatever the word is when Ligthroom CC 2015 asks you if you want to upgrade the catalog.
If you have your images in Lightroom 5 catalog, you just need to open the same catalog in Lightroom CC 2015.
Once you open the old catalog, Lightroom is going to upgrade your catalog from version 5 to CC and you can continue your edits from there.
You don't have to download folders to external drive as well as you can switch your work from Lr 5.7 to Lr CC.