Thank you (and others) for the suggestions. What I ended up doing was using the United website to search for one-way tickets from SEA to IAH on the date of travel, and sure enough, space opened up a couple of days ago. [As a side note/question, is there another, easier way to do this, or is the UA website the best place to go?] Once I saw the space in I/IN open up, I called the GS desk and the agent immediately changed the booking to IN. It was a very simple process with no questions asked.

One thing that I thought interesting (though it may be pure coincidence) is the space opened up exactly 5 months from the date of the flight. I'm not sure if there is a trigger that begins to open up award space at that time, but I thought it interesting in any case.

Again, thank everyone for the feedback and suggestions.