In the beginning of book 1 there are some situations with npcs and diplomacy rolls. I know that charm person will increase their attitude toward you. But what happens when the spell's effect wears off? Will they revert to pre spell levels?
This is more of a general charm person question than a WotR question.
When a spell expires, its effect ends, which normally means the NPCs attitude should go back to whatever it was before you charmed them.
However, some NPCs will be able to figure out that they were acting weird for the last couple hours: a good rule of thumb is the less friendly they were beforehand the worse they'll take it when they come out of it.
That is, if you charm a bystander into not summoning the town guard, when the charm wears off, he might think he was just in a good mood. But if you charm an angry orc who was about to attack you, and then get him to unlock some doors for you, when the charm wears off he's going to realize there is no reason he would normally have done that.
So, ultimately, it's up the the GM (just like its up to the GM how an NPC reacts when Intimidate expires or you commit a social faux pas.) I generally make it a practice to already be gone when my charms wear off.