How to reassign an order to customer’s alternate email

I have customers who, for various reasons, have created 2 or more accounts, often by creating one with an incorrect email. They then place an order for a downloadable product and later contact me that the order isn’t in their history (because they are looking at a different account.)

How can I reassign that order with the bad/alternate email to the correct customer account so that I can then delete the incorrect user account?

Most of these accounts were created in a previous shop system which was migrated to Woocommerce a few months ago. How can I prevent/reduce this from happening?

TIA!