The SD and HD locals are on completely different satellites and the spotbeam size is also completely different. On top of that, the size of the spotbeams even from the same satellite varies depending on the size of the DMA. The two Spaceway satellites that deliver HD locals use phased array antenna technology so can adjust the spotbeam size from very small to very large.
Also, just because you are in a spotbeam today does not mean you will receive signals tomorrow. Here in DFW, DirecTV just moved the DFW HD locals from the Spaceway satellite at 103 to a spotbeam on the DirecTV10 satellite at 103 (the first DirecTV10 spotbeam to be used, I think). As a result, several people in West Texas who had "moved" their service addresses to DFW and were receiving HD locals lost them completely.
You might notice I did not use the designation 103(a) for the Spaceway and 103(b) for DirecTV10. This is because the spotbeam transponders from DirecTV10 do not appear as 103(b), they appear as the higher transponders (above TP18) on the 103(a) screen. Just to confuse everyone.