2nd half of last year there was a string of poor weather and runway work in Sydney resulting in a real mess of delayed flights on multiple occasions. I was changed by DJ to earlier flights every time without asking, and was base red at those times and on the cheapest of the cheap tickets!
Obviously suited them more (suited me more too).
Last week, with cheapy fare again moved to earlier DJ flight (my request this time), done just as flight started boarding (with luggage). Full points for DJ check in guy who accommodated my request there. Other times, I've requested with suggested bright and cheery disposition, but been knocked back. Luck of the draw I'd say.
In the end, it doesn't hurt to ask. Just always be polite, remember they do have ticketing rules and it's us who's asking them to bend them just a tad!