It really all depends on any changes they made to the data services. If the new software connects to On Demand, Apps, and any other two-way services in the same way as the old software it largely doesn't matter. Or, if they can update things like Netflix separately from the rest of the system firmware. They can put the "old" software in maintenance mode, much like they do when a receiver is no longer in active development or just "finished". There's nothing that says they have to be actively developing the software as people are using it out in the field.
Just because older receivers don't get regular updates doesn't mean they are broken anymore. You can still get guide data for your 722 or older DVRs, right? Hoppers with the old software might not get new features, but they won't break.