Network share permission not updating
the icon overlays for the upgraded project appeared.
In conclusion, old projects using Subversion 1.7.x do not show icons and projects using Subversion 1.9.x do (under Windows 10).
It was NOT a problem with the naming of the icon overlays in the Windows Registry.
On Windows 10, most of the entries are used by One Drive and you won't have permission to remove them. Now give yourself "Full Control" and then apply it. As in current recommended answer mentioned, you need to sort the entries of the overlay identifiers in the registry.
In order to do so, right click on the entry (Example: "One Drive1", then click "Advanced", then click the link labled "Change" at the very top next to "Owner". I haven't deleted those One Drive or Google Drive entries but renamed all Tortoise Folders by adding 3 spaces to bring them on top.
Since I don't want to learn all about how that works, I am just viewing my work directories as tiles for now.
Windows explorer supports at maximum 15 custom overlay icons - they are shared between multiple applications (Google drive, One drive, Tortoise SVN).