I don't know if the EX's front brakes are the same size as Si's... I don't think so, right?

Anyway, if you want to switch to Si or teg rotors, you'll need the whole knuckle with calipers, brake lines, rotors etc. You cant just switch to larger rotors without relocating you calipers, so just swapping everything takes care of it all. ....and no, you can't just relocate your calipers somehow... thats why you gotta swap.