The EWC code can be changed after it has been invoiced and posted. This functionality can be found by
Hovering over Administration and clicking Admin Functions
Click the Transaction tab
Enter the Ticket Number in the Ticket No box in the "Update Transaction Items" section.
Click Search
Select the transaction items you wish to update
Click Select Checked Items
Note: You can also select the Transaction Type if desired, however this may hide associated transactions (e.g. Credit) that you may also want to update.
This will then display the Transaction Item Level updates, including the EWC Code.
From here, click in the EWC box under "Switch Transaction EWC Code", find the new EWC code and click Change