Update Custom Form Field in a Transaction Record

To do this, a workflow needs to be created followed by a CSV import in order to trigger the workflow:

I - Create a workflow:
    1. Customization > Scripting > Workflow > New.
    2. Enter a Name.
    3. Record Type = Transaction.
    4. Select the type of transaction in the Sub Types field.
    5. In the Initiation section place a check mark on On Update.
    6. Trigger Type = Before Record Submit.
    7. Context = CSV Import.
    8. Release Status = Testing (change this to Released when you are done testing).
    9. Click Save.
  10. Click New State > Click Save.
  11. Click New Action > Set Field Value > Parameters > Field = Custom Form.
  12. Value > Static Value > Selection = Select the custom form that you want to assign (Make sure that this custom form has Store Form with Record checked in the customization page).
  13. Click Save.

II - Perform a CSV Import 

Make sure to try this on three or less records to test, when testing is done then that's the time that can proceed on large records.

    1. Create the CSV file that only contains the Internal ID of the transaction records you want to update the custom form.  In the CSV file you will only have one column and that is the Internal ID.
    2. Once the CSV File is created go to Setup > Import/Export > CSV Import Preferences > Place a check mark on Run Server SuiteScript and Trigger Workflows.
    3. Click Save.
    4. Go to Setup > Import/Export > Import CSV Records.
    5. Select the Import Type which is Transactions.
    6. Select the Record Type.
    7. Click Select to select the csv file.
    8. Click Next.
    9. Click Update.
  10. Click Next.
  11. Make sure to map Internal ID to Internal ID.  (Note: You should only have one mapped field and that is the Internal ID).
  12. Click Next.
  13. Click Run.