As I understand the ATSC spec, the primary channel (.1?) must be MPG2, so even if stations started sending MPG4 on subchannels, the boxes wouldn't be totally obsolete. This seems genius to me - it builds in major incentives to move people to more capable devices without forcing anyone to change.
And as I understand the spec, most (all?) of the error correction is built into the transport layer, so if you can lock onto a signal carrying MPG2 data, you'd be able to lock equally well onto that signal carrying MPG4 data - just like today if you can lock onto any one subchannel, you've locked onto all of them.