Jon,

The sum of the probabilities must be 1.

The probability that your first choice is the winner is 1/3.
The probability the the cup raised by the house is the winner is 0.
The probability that the other cup is the winner is (1 - (0 + 1/3)) = 2/3
OR
There is a 1/3 chance to win if you stick. That is your original guess was a selection of one cup from a possible three cups.
The house will always disclose an empty cup from the remaining two thus, you now have a choice of only two cups.
Your original choice (1/3 chance to win)
or
the remaining cup 1 - 1/3 (ie 2/3) chance to win.

AndyG