Lightroom CC will be installed in addition to the Lightroom you have been using. Lightroom CC will offer to create a new upgraded copy of your catalog that conforms to the new version. But the old catalog and Lightroom 5 will remain intact and usable. Lightroom CC will be installed locally on your computer, and will function completely normally. If you decide at some time in the future to cancel your subscription, the develop module will no longer function in Lightroom CC. You will be able to view your images and export copies, but you will not be able to make any adjustments. The catalog that you have been using in Lightroom CC will not be compatible with your older version of Lightroom. So all the work that you have done would be lost. But you would still have your images, and you can work on them using your older version.
You won't loose anything you've done and your LR5 is not touched anyway and will run with its perpetual license in parallel.
If you buy a CC subscription for LR CC 2015, you can use the develop in CC 2015 as along as you pay the subscription. You can copy/upgrade your current LR5 catalog to work with CC 2015. However, be aware that you can not go back with this catalog and use it in LR5 - this is 'one-way' for the catalog and the work done in LR CC 2015.
Thank you so much!!!