The difficulty modifier in the chapter data is 2 bytes (4 nybbles) long. 1 nybble is the easy mode penalty, 1 nybble is the hard mode bonus, and the remaining byte is the normal mode penalty, so it can go up to 255. So most likely somewhere around 0x180FA is where you would have to make some changes. You’d want to swap the normal and hard modifiers addresses, as well as make normal be read as a nybble and hard as a byte.
Vesly made an autolevel patch that is meant to be used for enemies, which is an alternative you could use to do it through events instead, but there would be the drawback of having to do it every time new units are loaded, which would require different unit/class IDs otherwise you’d also autolevel units that were already autoleveled.
The default value is OFF, with the change it’d be the one in ONN. So to apply it through EA, you’d do:
ORG 0x0180FA
BYTE 0xFF 0xF7 0x63 0xFF
EDIT: Looks like I was mistaken in my first post. Apparently, the full byte could be read as the normal mode modifier, not for hard, so that would be another change you’d have to make.