I'm not sure why you'd ever want to go down a size, but FYI, often 6.5" speaker holes are the same (along with mounting screw points) as those for 5.25" speakers too. (This is usually more helpful to go the other way)
~2005 Disco 3 "Moneypenny"
~2008 GTI TSI
~If you can read this, thank a teacher. If you are reading this in English, thank a Veteran.