Hi Ariel,
There were a couple reasons we switched over to the New version. Primarily, the terrain on the Old map, as well as most of the builds, were very low-quality and unrealistic. If you warp to any places on the Old map, you'll see that most of it is very boring and looks like vanilla minecraft. There was very little variation based on climate and region. Secondly, we developed the custom launcher, and added a large amount of custom blocks to the game. For both of these reasons, it made sense to redo the map using WorldPainter. We copied a couple builds over (like Harrenhal), but decided to redo most of them.
The main difference between the Old and New map is mostly the quality and the types of blocks used. I believe that the scale of the New map is also larger than the scale of the Old map.