phases will require phase filter settings that differentiate what is existing and what is new. Your walls will be created in different phases (i.e. existing, or new construction).
Your elevation view properties should be set to show the new construcion phase, with a phase filter of "show previous + new".
In the phases menu, set that filter to use a graphic overide (the details of which you can set on the overide tab of that menu) for the "previous" phase elements (i.g. existing walls).
Those settings will affect the plan views mostly, but using shading with edges will produce an elevation with the previous phase shaded in gray.
(at least that is how I've gone about in my project. good luck).
