For any customer quotes that are done in the Quote and Invoice Management screen, as they are not final, they can be switched to a different customer.
This may be needed if the wrong customer name/account was placed against it, or they've arranged for a different person to accept the quote.
Pro-formas, even though they cannot be edited, can be reversed back to a quote, essentially letting you edit them.
If a transaction has already been processed to become an Invoice or Sale, it can no longer be edited.