It seems a number of people have been successful in transfering their old game files...just not us, I suppose. I'm sort of relieved I'm not alone, though.
I've found that if I only replace the folders for Characters, Lots, Storytelling, and Thumbnails the default neighborhood of Veronaville will still show up when loading the game. When I enter the neighborhood, some of my Sims replace the Shakespearean ones when I click on a lot and see the family posed together. When I enter the lot, the Sims turn back into the default ones, but the houses are mine...furniture and walls as I had placed them (any additional lots I created don't show up on neighborhood view).
This only happens with the central neighborhood and not Downtown or Bluewater or anything.
Veronaville only seems to disappear completely when I replace the N003_Neighborhood.package file with mine.
I tried uninstalling UC and reinstalling with my discs, but my computer wouldn't let me install Best of Business or Fun with Pets, saying they were already installed (they're not...I removed everything per EA's instructions including registry keys). So I hit a wall there as well.