To be fair to TfL staff, I've had to do this several times and only once did the staff member put in the wrong discount.
I'm not sure of the exact process, I know that the staff member uses a special 'staff only' menu on the ticket vending machine to select the appropriate railcard discount and enter the expiry date.
The important thing is that, after getting the staff member to load the discount, tap the card on a ticket machine yourself and make sure that it says 'Disabled Railcard' rather than 'National Railcard' at the top.
If it doesn't, then go back and get them to fix it.