The odds are around 2% to 3%, given that around 50-60 awardees reject their offer every year. I got an HM too, with two very positive reviews (E/E) and one reviewer who didn't like my GPA, but otherwise really liked my application (VG/VG). I think it's best not to think about it... But I'm sure half of the HMs will worry about it until the upgrade letters go out anyway (mid-June I think).
Oh: and long-time reader, first-time poster.