Once you have passed the MQ with a class on your character, you can attempt it again. However, even if you choose the correct keys and technically pass, it will still count as a failed MQ and only award 1 guild point. The message that all online users will see in the chat will indicate that you failed the MQ, regardless of whether you selected the correct keys. This is what you would see:
Passing the MQ with the same character as a rogue (to unlock samurai) will only be possible in the next season.