Let me make sure I understand your problem. You moved a Sim out of your main household and now you want to play him instead of the main household? To do this, you need to follow these steps:
1. Click on the menu button in the lower left corner of your screen while you are ingame to go to the Main Menu (this button has three dots on it).
2. Click on Edit Town.
3. Once you see the Map View of your town, there will be a button with two little houses on it in the lower left corner of your screen. This is the Change Active Household button.
4. Click it and you will be able to select another household from the map.
5. Choose the house where your new Sim lives. You'll get a warning message that changing your active household will cause you to lose the promised wishes of your current household.
6. Continue on through that and you'll then be in the house with your new Sim and he will be under your control.
Be aware that if you have Story Progression turned on, your main household will continue their lives without you. You could go back to them and find that they have changed jobs, gotten married, had children, or any number of things. To turn Story Progression off, click on the lower left button for the Main Menu and choose Options. One of the tabs will have the Story Progression option.
I hope this information helps you.