The Import feature DOES NOT have the capability to UPDATE constituent preferred addresses, this is done via the NCOA utility which was designed to update address cards marked as PREFERRED and update only the actual address fields without disturbing the other information on the card.
The only way to use the Import feature to change the preferred address information would be to REPLACE (Delete) the existing preferred address card with a new preferred address card.
BUT and this is the main caveat, you would have to know the Address type of each individual constituent's current preferred address card AND since the card is being replaced (deleted) you would have to ensure you have ALL information from the current card in the data file to be re-imported into the new preferred address card you are creating.
That means, information contained on any and all fields on the exiting preferred address card would have to exported out of the database, then the address and any other field you wish to update is then changed. The address TYPE must also be included in the export file as this is the only way to ensure you are replacing the existing preferred address card with the new preferred address card.
Once the data extracted from the database has been edited, you would need to add a column to the data file titled Preferred and enter a value of Y in this column for each data row. The second column to be added would be the Address Type column where you enter the address type of the existing Preferred address card.
Then and only then would you be able to replace (delete) the current Preferred address card with a new preferred address card. Remember the system is replacing the ENTIRE card - the current Preferred address card is being deleted and a new card is being created. Again, any information not exported out of the database from the existing Preferred address card would be lost.