For various reasons, I've ended up with more Pay As You Go credit on my Oyster card than I'm now likely to use in the next 18 months. I'll still be using my Oyster card, so I don't want to cancel it for a refund+deposit, but I'd quite like to get some of the credit back off of it.
The TFL Oyster Refund page is a little ambiguous about this case. It mostly talks about cancelling the card for a full refund + return of deposit, or for refunds of season tickets / travel cards loaded onto the Oyster. However, under the Refunds at Ticket Offices part, it does mention
Refunds of pay as you go credit
Is it possible to receive a partial refund of Oyster Pay As You Go credit? Or is the only way to do it to cancel the card for a full refund, then get a new card and only put some of the money back onto it?