We have no official support for moving or changing tickets yet. However, it is possible to move the tickets that a customer bought by refunding the customer's tickets and asking them to purchase new tickets for the correct screening/seats/ticket type.
Follow this article to refund the tickets of the customer: How to refund an order
Ask the customer to buy new tickets.
This method can also be used if the customer has bought the wrong amount of tickets, or the wrong ticket types.