The original 2003/04 EVO 74 is still a very good board. In a way you could almost say it is better than when it was introduced since it is better understood nowadays. My point is that it is a very good board you have there and you don't need to change just for the sake of it. It took many years to come up with something better (the change to round tail in 05 was very minor)
That said, the 09 (and 08) EVO 75 XTV has been improved by mainly a better v layout and then some slight adjusments to the rest of the shape. This may not sound like much, but it adresses the particular area where the original EVOs had their biggest weakness in the wave riding in my opinion. That is a fast carving cut back. With all that tail, you had to set up such a turn very carefully not to slide out of the carve when you applied pressure. The new XTV is more natural when entering such a turn.
This may sound like a technical detail, but it translates into more control and simply a better behaviour in faster wave riding, meaning the board has a wider range. In practice you don't loose any of the good things in slow wave and light wind, but you gain some range in higher winds and in faster waves and side shore.