1) Mojang is planning to allow Minecraft username changes with the ID changes coming in 1.8 and beyond. They've not yet announced /when/ we can expect this service to be live, but apparently it will be free with some limits so you can't just bounce between names as you please. Right now, the only way you can change your username would be by registering a new account.
2) It doesn't appear that the change will affect worlds too badly, in that worlds will automatically update players in the session both online and offline. However, players will be expected to update their servers to accommodate the new format as the old format will become unsupported.
3) It's intended to be released as a free service, yes. But exactly when isn't currently known.