I have Lightroom 5 installed on my computer. If I purchase a Creative Cloud version does that mean I will have two copies of Lightroom on my computer? Can they share the same library at the same time? If I later decide to end my Creative Cloud contract, can Lightroom 5 be used with the library that was used by the Creative Cloud version and would it read the photos that were modified with tools only available in the Creative Cloud version?
I haven't gone to the Creative Cloud yet. However, I can tell you that Lightroom is the same program regardless of whether you have Lightroom 5 standalone or Lightroom 5 CC. The only thing that's different is the licensing. You will have to uninstall Lightroom 5 in order to install the version from the Creative Cloud. You will still be able to use the same catalog that you have been using, and everything should run normally. Lightroom will continue to be upgraded. Eventually I believe there will be a Lightroom 6. Historically, each new version has required a conversion of the catalog to a new updated format. If that happens to your CC subscription, that new catalog will not be usable with Lightroom 5. So you will have to start over at the point you left off. Other than that, you should have no problems.
I just happened across an article published by Adobe that explains what will happen if your creative cloud subscription ends: