Well, it happens on KE, too. But in a uniquely different form.
not from Y to C, but Y to "the crew rest area"

I was flying KE in Y on an A330 from AMS-ICN last year.
My colleague, who had an operation a month before, felt really out of sort and became ill.
I called for a female purser and asked if she would let my colleague to sit in C as I had noticed the low load factor there.
Her answer was crystal clear and blunt "no".
"Sir, upgrade on board is not allowed"
To which I retorted, that the plane might be forced to make an emergency landing in Siberia, if my colleague's condition became critical.
She said " if that situations comes, we will land in Siberia, Sir"

"But, as upgrade on board is prohibited, I can offer your colleague the crew rest area as a humble alternative." She was friendly, yet very clear.

My colleague eventually slept in the crew rest area for the rest of the flight, and he was as fresh as hell by the time we landed in ICN.