It does overwrite SB and non-SB illusions, just not all of them. Credit goes to Finster for figuring out that this is probably determined by the illusion's required level to cast. So for instance Gnome will overwrite Dark Elf, but not Iksar or another illusion higher level than gnome.
I'm for making the earring simply not overwrite any existing illusion.
I'm for making the earring simply not overwrite any existing illusion.
