what do you mean by having a normal and a hard mode instance? are we talking about 2 different instances with different monsters/story/etc..?
if that's the case then it's fairly simple, you want you can look at Old Glast Heim's entrance npc for example:
.@mode = select("Normal Mode:Hard Mode");
if(.@mode == 1) {
// enter intsance #1
} else if(.@mode == 2) { // Hard Mode
//enter instance #2
}
but if you have only 1 instance and want to make a hard mode from that instance well then it depends.
if we're talking about a single instance you can just store the difficulty in an instance variable and edit the monster spawns and summon a harder version of the monsters in hard mode, for example:
if('OGH_Mode < 2) // Normal Mode
monster 'map_name$[1],158,255,"Amdarais",2476,1,instance_npcname("#ghmemorialmob07") + "::OnMyMobDead";// MG_AMDARAIS
else if('OGH_Mode == 2) // Hard Mode
monster 'map_name$[1],158,255,"Amdarais",3150,1,instance_npcname("#ghmemorialmob07") + "::OnMyMobDead";// MG_AMDARAIS
but yeah it really depends on what the difference between normal and hard mode is.