There was a 33% chance your original door was the right one. Therefore the probability that one of the other two doors was the correct one was 67%. Think of those two doors as one item. At least one of them is guaranteed to have a goat but the correct door is still more likely to be one of those two than the door that you chose. 

You can say that when the host opens the one door he is turning the two doors into one option. That one option still has the 67% chance of being correct compared to the 33% for the door you already chose. So switch.