Can you delete LR 6 after upgrading to LR CC Classic? What is the downside to deleting the older version? Seems it is just taking up disk space.
Yes, you can delete it. I see no downside.
Lightroom Classic CC is not dependent on Lightroom 6 for anything. If you are certain that you won't be needing it for anything, you can confidently uninstall the program. You might want to store your serial number in the event that you decide to revert to it at some time in the future. But be aware that the Lightroom Classic CC catalog will not be backward compatible with Lightroom 6.
Uninstall, not delete.