With the latest patch that was installed with Island Paradise, you got the option to move your Sims to different towns, purchase additional homes, and build your homes on the water. Basically, you go through the motions of moving your Sim to a new home within the town, but there will be a new option - move to a different town. It will cut all connections with Sims in that town and your Sim may lose their job if the new town doesn't have that job available. And yes, you do need at least 20,000 as a 'down payment' for the move, which will be returned once your Sim gets to their new home.
Hope this helps!