Benefits of Upgrade from 20-700

Our current HDTV DVR 20-700 has to date provide excellent satellite and OTA performance.  Since the Dallas market has so many quality OTA channels OTA capability is important to use especially during satellite down time (due to weather).  My understanding is if I upgrade to a newer HDTV DVR a seperate component is needed to received OTA stations.

Questions: 

1. What benefits would we receive by upgrading to a new HDTV DVR?
2. What is the line item cost to upgrade to a new HDTV DVR with added OTA receiver?
3. Could we just puchase the new HDTV DVR and still us our current 20-700 to receive OTA stations?

Thanks,

Louis