I had a SAN-SFO flight change from 12F to 8F right around T-48h. I bought F about one week out and noted that the cabin was sparsely filled (maybe 6/12). I avoided row 3 and grabbed a row 1 seat... have there been involuntary downgrades due to this issue (don't really remember seeing stories here), or are they doing this on flights with predicted lighter loads only?

When we were hearing about downgrades when a lot of the 739s were being subbed for 738s, it seemed like row 4 always got the shaft... downgrades weren't done by status or paid vs upgraded tickets, but by whether you were unlucky enough to happen to have a seat in the row that was removed.

On the other hand, if they're generally waiting until T-30-36h, as others have mentioned, they probably have a pretty good idea of how many seats will be filled by then.