On the blending note - just out of curiosity I decided to strip a copy of BP down to its core chunks and load it up into PreRelease2 to see what would happen. Pretty interesting!
The blending doesn't blend different ground type so much as it alters the levels and ground shapes of the new chunks to fit in with the old chunks. In one of the pics there's a village that was expecting to be on the side of a mountain, but the terrain had to roll down towards sea level instead.
In any case, the old builds seem to be staying intact.