I've had a whopping 3 as an EXP. One was never explained, the other two were C to F upgrades during OSO/reroutes. The EXP desk has told me they try to fill the F cabin when protecting EXPs when they are dealing with OSO.

Now, I've been downgraded from an A ticket to Y before (originally booked as J eVIPed to F) and from an A ticket to Y mid-flight. Those cancel out.