I was able to upgrade Font Awesome to version 6 after editing the extension. The upgrade went mostly ok, with icons being replaced with their updated counterparts.

However, as described by the aforementioned guide, for this to work three CSS files are required, fontawesome.min.css, solid.min.css and v4-shims.min.css, in this order. After the work done for XWiki-15149 this can no longer be implemented in the extension and should be called directly from the jar.

The component TreeView had issues with the upgrade. This component implements a copy of FontAwesome 4 classes in its own stylesheet (but not the font files), with class names pointing to Unicode characters used originally in the v4 stylesheet ("jstree-icon jstree-ocl", for example). These class names should be refactored to use the ones from FA properly. 

It is important to remember that Font Awesome don't recommend importing two font files from different versions of the project. So calling v6 for the interface and v4 as a fallback for the tree view to work is not something that should be done.

Some screenshots after the upgrade

The problem with TreeView in the Navigation pane