Parisis,
Depending on where you live, it probably is more than likely that Comcast boosted/improved the signal when they rolled out HDTV. We had the same experience here back in August when it was rolled out in our area. Before this occured, I used to have a signal amp right where the cable signal came into the house. After the upgrade, my signal was overamplified - it was obvious that the signal strength from the street had improved. More evidence that something had been upgraded by Comcast was that the week before the rollout, we saw Comcast trucks everywhere in town working at the poles.
With that said, what Jack said is also correct. The signal quality of individual stations can vary day to day if not hour to hour
cheers,
--tom