Was it a manual recording (i.e. you specifically set the recording) or an autorecording wishlist? Pre-Premiere, ARWLs prefer lower #d stations, and on the Premiere, they seem to prefer higher #d stations.
If it was a manual recording you did, I'm not sure what happened, though if it somehow thought the channel went away, it can reschedule them.. that's highly unlikely though.
If you NEVER want to record the SD channel, you might want to remove the SD channel from your lineup as a lame workaround.
I have most HD channels removed so that my various ARWLs don't accidentally catch a show on a HD channel and cause a bunch of shows to be nuked.