I can't tell you what the penalty is, they used to show it when you go to a mock booking. What I can tell you is that it's different depending on the destination. If it's a domestic flight, best guess is it's a bit cheaper, but a long haul or flight to the US it's probably maxed out.

I'm not gonna look, but you can if you like. Make a mock booking, when you go to pick your flight, highlight over the different booking classes and you SHOULD find the info. It's been a while since I've done that so they may have changed their IT since then, but I remember I had to change a domestic flight once, but the flight was so cheap I just no-showed it instead.