By the way, when they say it downgrade the system settings, it should still be displayed "9.x" and not "4.x" right?
Because on my N3DS, sysand still display "9.0.0-20E" but as the exploit is working, I suppose it's the normal behavior?
First installation have to be done "manually", though (NVRAM option is the gateway menu, then the "mset.cia" file on sysnand). Just installing the .NDS file is not enough. Once it's installed and if the exploit is removed (by playing DS game), the .NDS installation should be enough to reinstall it (though I didn't tested it yet).
And for people who haven't unlocked the Blue Card yet, also need to install the "bluecardfix.cia" on sysnand when also installing the "mset.cia" file.