What happens when you change the price of a product which already has orders?

What happens when you change the price of a product which already has completed/current orders?

Example:

Product sells for $40.
Customer A buys and pays for the product.
Customer B has ordered the product but payment hasn't been processed yet.
Product's price is changed to $50.

Does Customer A's invoice show the old price? Does it indicate the price is different than the current product pricing?

Does Customer B's invoice remain the old price?

Thanks!