Ask HN: Pivot/idea change - where do you bury the survivors

For founders who were accepted to YC after launching and gaining traction with one idea only to pivot or completely change ideas, what did you do with the users/customers from your first idea? Especially in cases where the new idea is completely different, do you just close up shop and let users know you've moved on? Do you send them to a competitor? It seems like such a shame to see that work go to waste.