It appears that if you have a font set up in mIRC.ini that is not installed and you run mIRC, it will change the font as expected. However, it also will remove all custom Color Schemes as well and revert to mIRC Classic. It may also change other settings in the mIRC.ini file, though I haven't noticed other changes. It makes sense to change the font since it's not installed, but nothing else in mIRC.ini should be changed.