Can I open my old NXT program files in the new EV3 software so I don't have to rewrite the entire program?
No.

Unfortunately there are no shortcuts. EV3-G can not open programs written in NXT-G.

To quote Lego:


  9.5 I have a LEGO MINDSTORMS NXT2.0 (8547) set; will there be an upgrade/conversion kit from NXT to LEGO MINDSTORMS EV3 (31313)?
  No, but LEGO MINDSTORMS EV3 offers backward compatibility to NXT. You can therefore use your NXT Intelligent Brick together with Intelligent EV3 Bricks in many situations. However, if you want the full benefit of LEGO MINDSTORMS EV3, you will need to transition to Intelligent EV3 Bricks.