If you add a CityIDAtEntry column to the table as you suggest you can then automatically insert a value into it when you enter the first address for the person concerned by means of code along the following lines in the AfterUpdate event procedure of the CityID control:
If IsNull(Me.CityIDAtEntry) Then Me.CityIDAtEntry = Me.CityID End If
The value in the CityIDAtEntry column will remain static when any amendments are made to the CityID control by virtue of it not being Null after the first execution of the above code.
Note that the values stored in these columns should be long integer number foreign keys referencing the autonumber CityID primary key of a separate Cities table in an enforced relationship. City names are unsuitable as keys as they can legitimately be duplicated. For an illustration of how to store address data in a set of correctly normalized tables take a look at DatabaseBasics.zip in my public databases folder at:
In this little demo file the sections on 'one to many relationship types' and 'bringing the model together' illustrate a suitable model for addresses. In the section on 'entering data via a form/subforms' a contacts form illustrates how such data is entered, using a bound combo box for the CityID and unbound combo boxes for the region and country in which the city is located.