If you are attending your first event ever, afterwards you are allowed to change your character and not lose any of the Experience Points you earned at that event. This is only done for your first event as a player — not your first event as a new character.
For instance, Terin attends his first event as a fighter. After the event, he realizes he would much rather be a spellcaster. He is allowed to re-spend his Experience Points, but after that he can never change any of his characters again this way.