I tried searching, but it seems my problem is a bit difficult to explain and therefore difficult to search for.
A couple of months ago I moved my WordPress install from the root directory into its own subdirectory, following the instructions found here. That means my site's root is currently powered by WordPress, using a static page as the front page when visiting my site's root, and that the only WP file in the root is the index.php.
However, I now want to use another, non-WordPress index file for my site's root. In other words, I want to move the index.php back into the subdirectory with the rest of the WP files, leaving it possible to use a different index file for the root. I just can't figure out how to do this successfully. I tried just moving the WP index.php file into the subdirectory where the rest of the WP files are, but I'm too confused to figure out what I need to change in the General Settings to make it work (if it's even that easy?).