In the PS3 version, you choose "Move" on the phone menu.
If you have enough money, you can move to a new house and choose to either choose to have that sim move in or leave him/her behind.
Note: Please note that sims left behind will be deleted. It may be different for the Wii version, though. I'll have to buy it and play it.
Hope I helped!
I'm now in love with the little "despicable" Minions from Despicable Me. I saw the second film a few weeks ago and loved it and now I adore the Minions as well. They're too cute! Who doesn't love Minions? And I might not have a new sig and avy up for a while since I'm saving Images to my 3DS Camera. Look on Examiner.com to read what they're saying in the first film's credits.