There is no set date, but it is certainly years off. Once all transmissions are MPEG-4, millions of older receivers will be obsolete and no longer work. Replacing them will not be a minor expense. Best to spread that expense over some years. Of course, the bandwidth recovered will be of great value, too - delaying or eliminating the need to launch another sat.
That's already happening. It's rare to see a 322 on a work order anymore. They're almost always 222s. Doesn't matter if the customer is getting HD or not. And single tuner non DVRs are always 211s now. We haven't carried 301s or 311s in quite some time.