1) Check the ID of the action called when printing the delivery slip on the transfer

This calls the report of the delivery slip matching this ID 

2) Get the external ID of this report (the goal will be to change the external id from the standard report to the new one and obviously vice and versa)

On the report => put debug mode => click on the small bug on the purple bar => view metadata

Copy this part of the external ID

3) Go in the menu of external Identifier 

Search for the record with the external ID you just found

Here you can see that it is calling the standard delivery slip with the ID 791

4) Change the value in the record ID with the id of your new report (here 530 for example)

5) print the barcode sheet to be able to use the print delivery slip

6) GO in the barcode app and when needed scan it => Odoo will print the new report (here id =530)