I feel like I am really missing something here. I don't see how the modal argument is supposed to work. I have lots of evidence that I am conscious in this world. But how is that evidence supposed to help when I move to a different world -- one in which I may or may not be a foobar?

At a first pass, I just don't know how to parse the claims you are making. Are you saying, for example, that P(I am a foobar in this world) < P(A foobar is conscious in this world), or P(I am a foobar in some possible world) < P(A foobar is conscious in some possible world), or ... ?

At a second pass, I'm not sure how to evaluate the probability of modal claims.

At a third pass, I'm worried that your argument equivocates on the interpretation of probability in your two assumptions. The first assumption -- that P(I'm a foobar) > P(A foobar can be conscious) -- seems to use a modal relative frequency interpretation: where the probability of an event is the frequency of possible worlds in which the event occurs. The second assumption -- that P(I'm conscious) is nearly one -- seems to use an evidentialist or maybe personalist view of probability. But I don't think these two can be combined unless you have some principle by which evidence that I am conscious in this world is also evidence that I am conscious in nearly every possible world.

Could you try explaining in more detail?