There was no "HDTV switchover date". There was a digital television transition, and it occurred on June 12, 2009. Full power analog stations are no longer on the air, the transition accomplished what it was supposed to: end analog over the air(not cable) broadcasting and replace it with a digital channel.
The SD/HD channels you speak of I assume are on a cable or satellite system. The 6/12 DTV transition had nothing to do with cable and satellite TV, as they are not broadcast over the air. Cable and satellite TV continue to operate the same as they did before, at the provider's discretion. Cable and satellite continue to carry both SD and HD channels because of equipment limitations. Cable TV has analog 'must carry' rules on local channels and both also have many customers with SDTV sets and SDTV equipment that would need to be replaced to carry HD only channels, like OTA broadcasting.