Buy-ups work with any booking class, while upgrades "purchased" with miles are permitted only for certain booking classes. My wife was offered a buy-up from Y to C on a SYD-LAX flight (for $550, I believe), but was not permitted to use miles for the upgrade because the ticket was not M or H class.

Not to go too far OT, but does anyone have a sense for how frequently Mr. Chicken offers buy-ups? I'd love to snag one for an upcoming NRT-ORD return. The westbound flight isn't bad, but I'd love to be able to get some sleep eastbound. [Edited to add: I know there's a long thread reporting experiences with buy-ups, and the offering prices. I was just curious whether anyone knows whether these offers happen all of the time, some of the time, only when Y is packed and C is thin, etc.]