To expand upon Rad's answer, there still would need to be several billion dollars spent on replacing dishes and receivers before everything could go MPEG4. Ain't gonna happen for a long time.
How long after bringing out the MPEG2 HD DirecTIVO did they wait before they obsoleted the entire line? Seems there are some still angry about that.
The current boxes are available that output composite AV and S-video, along with HD. Legacy SDTVs can be fed a signal from the current HD boxes.
While we talk about the equipment change out costs, that ignores the cost of maintaining the SD signals. Channel providers are starting to only provide HD signals (e.g ESPN). This means lower distribution costs for the provider. You would only need to uplink one signal and you wouldn't need to have the equipment to downconvert the signal and send it up for distribution.
My bet is DirecTV has a number of people running models and comparing operating costs and revenue projections for various scenarios. They are probably closely tracking HD vs SD dish installs (one HD receiver requires an HD dish), and ratio of HD to SD receivers. They can probably give pretty accurate predictions of when 50% and 75% of new install receivers will be HD. The also know the rate at which SD receivers are being replaced by HD ones. Since joining DirecTV 4 years ago, I retired 2 of 3 SD TVs and replaced them with HD TVs. The last SDTV is in a guest room and never watched (something else I am sure DirecTV can track). Surely, they can also give a decent date estimate of when the number of operating SD receivers will fall below a threadhold making separate broadcast no longer cost effective/profitable.
The question is whether, when they need to contract for, and begin construction of a replacement of the current SD satellites, will it make sense economically to continue to broadcast SD signals. Given that multiple satellites are involved in SD delivery, I could imagine a scenario where they slowly reduce services available SD only (e.g movie premiums and/or PPV require an HD receiver) during the transition period. This way, they could wean SD customers from their SD equipment without forcing people to switch all at once.