I assume, then, that the new cap comes into force immediately?
Yes, it seems to have done so.

For anyone intending to go through this process, let me just let you know
what my experience of it was today.

First up, West Hampstead Overground station - where the cashier simply said
"I have no idea how to do that, you'll have to go to the Underground
station". And that was that.

So, I toddled off to the underground, and was told that it wasn't possible.
I tried to explain that it was and even showed the relevant page in the
Tickets and Fares leaflet. They still didn't relent, and after telling me
that I would have to hold up everyone else in the queue while they checked,
still refused to do anything other than sell me a normal travelcard. I
argued the point some more, they did some more phoning, and decided that it
*could* be done, but I would have to get a brand new oyster card to do it.

So, new oyster card in hand (and another £3 deposit paid for the privilege)
they added it on. The whole palava took 15 minutes and as well as appalling
customer service from the staff (who in effect told the rest of the queue
that I was holding them up by being unreasonable) I was also subject to
heckles from other passengers.

Moral of the story? If you want to attempt to add your railcard onto your
oyster be prepared to argue your point, have the relevant page in the
leaflet to hand, have plenty of time to spare - and I would guess try to use
a station with multiple ticket windows. It's not an experience I want to go
through again.

Best Wishes,
LEWIS