I love the Orion design. It's simple, very rugged, and is actually doable with real world technology. No magical warp drive or hyper space needed. On the downside, Orion is fairly messy. It's pulse units produce lots of debris, and I would wait ten years in between launches just to make sure the path was clear for the next ship. I wouldn't want a ship to get hit by any relativistic debris from the pulse units of the ship that went before. That's probably paranoia on my part, but I'd prefer to play it safe with a multi-hundred billion dollar craft.

Another more plausible reason would be to take advantage of the technological advances you mention. You would want a good deal of the technology to be the same as the previous ships just for the interchangeability, but some advances (medical, power production, etc) would be too good to pass up.

Concerning propulsion systems, the rockets we use today aren't very different from the rockets of the 1950s and 60s. How much interstellar propulsion systems would change over a period of a century is up in the air.