Can I Request Presenting a Different Paper at a Conference?
At humanities conferences it is normal for people to give papers that are nearly unrelated to the abstracts that they submit. Few people who change their paper topic bother to notify anyone about it. I admit that I've done this a few times. It would certainly be polite to notify them that you are going to speak about something different, but there's no need to feel bad about it or be apologetic. This is a normal thing. No one will mind. 

As an aside, you'll soon notice that many senior academics provide extremely vague abstracts to conferences. This is because they don't know what they want to talk about, won't know until they make their presentation on the plane there, and want to leave their options open.