I also ran into this, don't know if there is a reason it is intentional, a technical limitation of some type, or just overlooked and so a bug.
For AC 14 and earlier I used an Empty Fill (copied and renamed Existing) for my existing walls (and slabs, roofs, etc), and had used a blank white Solid type fill. For AC 15, to utilize the Renovation override feature, I copied a nearly blank Vectorial Fill (like Cut Stone), increased its spacing to infinity (ok, 328083'-11.874" which is some ArchiCAD limit) and named it Existing. This has worked for me so far in terms of getting a result that matches my previous standards while being able to use the new feature. I don't know if there is a similar solution for getting a black or % fill from a Vectorial Fill to match or simulate one of the other Solid Fills.
David