If you want to update the Preferred Address for existing constituents, then you will need to set your template accordingly. This can be done with either the Default Value on the Preferred field of the address mapping or the Address Area Settings,
To do this using the Preferred field Default Value, you will need to change the default value to a checkmark (shown below).
If you choose to do this via the Address Area settings, you can opt to select "Always updated Preferred Address".
With either of these choices, you can save the previous preferred address (see this knowledgebase article to learn how).