The System file's name cannot be changed because the "System" bit in its Finder information section has been set.
You can set the "System" bit of ANY application using ResEdit (for this example ResEdit 2.1.1 was used):
1. Run ResEdit.
2. Select "File/Folder Info..." from the File menu.
3. Find the application and open it.
4. Find the checkbox to the right of the application name on the top line.
5. Click once in the box.
6. Click in the Close box of the window.
7. Click on the "Yes" button to save the settings.
8. Repeat steps 2 through 8 until all the applications are changed.
Note: If a file with the "System" bit set is duplicated using the Duplicate command in the Finder's File menu, the new file will not have the "System" bit set.