Moved from Lightroom CC (Cloud) forum.
First of all, you cannot "purchase" Lightroom Classic CC. You can subscribe to it, but you cannot purchase a license out right. Lightroom Classic CC is part of the Creative Cloud photography plan which includes Photoshop among other things, and the subscription fee is $9.99/month in the US. There is a discount for paying annually.
But to answer your question directly, you will still be able to continue using Lightroom 5. After installing Lightroom Classic CC you will be prompted to allow it to upgrade your catalog to the new format. This process will create a new copy of your catalog that will only be compatible with Lightroom Classic CC. However, the original catalog will still remain, completely unchanged and usable by Lightroom 5. The two versions are not compatible with each other. In other words, work performed in one version cannot be seen or used in the other. So I'm not sure how long you would want to continue using an older version considering those limitations and considering the feature enhancements you will experience in Lightroom Classic CC.