Yes, LR 6 will convert (by making a copy) your catalog, and you can then edit in the new version. You must take some caution, e.g. in operations that change your actual images (such as writing metadata or moving images around).
The safest thing would be to NOT convert your catalog but rather build a whole separate work area of images and catalog that is disposable, but it is certainly possible to test LR6 in place if you are careful and aware of what operations you take.
Note you can run LR 5.7.1 and LR 6 (aka CC) from Creative Cloud at the same time as well -- you can even go back and install prior versions from the CC app.
Lightroom CC will be installed in addition to whatever else you already have installed. It will prompt you to upgrade or update your catalog, but it will do that by creating a new copy. This will leave your present installation installed and intact and completely usable as it has been used before. Just remember that although you will be looking at the same images in the catalog in Lightroom CC, the adjustments that you make will not apply to your Lightroom 5 catalog. Your other alternative would be to instruct Lightroom CC to create a whole new catalog. Either way will work.