Software for the DSR704 and DSR708 is exactly the same (the only difference between the two models is the size of the original hard drive). Please also refer to posts like this, as well:

http://www.dvrplayground.com/forum/thread/11747/&highlight=digits

For answers to commonly asked questions, please refer to the
InstantCake Instructions and Release Notes,
Official InstantCake Discussion Thread, and the
Official PTVnet Discussion Thread

There is no way to change it. In fact, it may not show as DSR70X at all. On many units, running 6.2, it will show up as DSR/7000. This holds true for some of the Hughes units. DirecTV has a history of screwing up that particular field in their system information screen. In any case, ALL that matters (from a technical perspective) is the information in that previously linked post.

For answers to commonly asked questions, please refer to the
InstantCake Instructions and Release Notes,
Official InstantCake Discussion Thread, and the
Official PTVnet Discussion Thread