As duchies are purely abstract constructs anyways, I'm even thinking about allowing them to be changed rather freely. Maybe with a timer and a gold cost so it's not abused, but aside from that?
I don't think I'd allow free-form renaming, even at a gold expense. Let's face it, a gold cost, or a short civil-unrest cost is essentially no cost at all. There's enough gold floating around that it's a non-issue. What you'll end up is someone changing the name as soon as they get the duchy, or save up the gold.
