If you navigate to the Pledge in question and enter the Edit mode, both Source code and Destination should become available for modification under the Details tab.
You can also change the source via a Global Update by following the steps below:
Select all the pledges you wish to be changed.
Click More, then Global Update.
Select the Source in the Field to update dropdown.
Enter the source or destination code in the New Value box.
If payments have already been made, these will need to be reversed once the change to the Source or Destination Code has been made, and then recreated on the system.