I found it. When you click the "Update' button on Creative Cloud App, it downloads the new version and starts the install. On the first screen of the install (heading: "You're upgrading to Lightroom Classic CC"), there is an item lower on the screen that says "> Advanced Options". Click the spinner arrow to open the list. The only item in the list is a checked box labeled "Remove old Versions". UNCHECK THIS BOX.
Then hit the update button and the new LR Classic 7.0 will install without mucking with your older LR CC/2015.x version. in the process it will ask you if you want to convert your older catalog to the new version and will show you which one it found (looks like the last one used). If you say yes, it will create a new ".lrcat" file and a new previews folders with a "-2" added to the file/folder names (or higher number if 2 is already taken). These items will be in the same folder as your old catalog.
You will get a new icon on your desktop that I suggest renaming so it is apparent which version is which.