Having to require a discount reapplied to my Oyster recently, I was told by queue busters outside the Kings Cross Visitors Centre that staff inside would be able to do it only to be told by staff inside they actually couldn't so having wasted my time patiently queuing I wasn't impressed.
Thankfully the gateline staff were able to apply the discount for me thankfully!
(King's Cross) St Pancras visitor centre were able to do it for me. I asked about if they knew anything about when you'd be able to load a Railcard on to a contactless payment card and was told that would happen at the relevant bank!