The conversion would require the whole front end
Fortunately because you are going from same frame to same frame you won't have to build a custom top head, or custom bolt to connect the front end.
You wouldn't have to replace bearings just get the:
Headset top and bottom
Front forks + hugger and mounting brackets
The wheels
Rear shock ( I don't know the difference between ds and s) my guess is the fx is the rear shock in the ds, only because they used the same forks from fx to ds, only difference was the bottom shoe despite the ds being easy 100 lbs heavier than the fx
15' is the big step up in suspension
To give you a comparison, a fox rear off-road shock made for a fx will perform better than any fast ace stuff made for the street...(on the street). . But once you get the real street Showa stuff, there is no way anything will compare fox, or not...even the 14 (full size) fast ace suspension is much better than the 13s normal small tube forks
I'm trying to cram showa rear on my 13' fx "road racing bike" but I'm going to have to make custom holders to move the shock 1-1.5" back to adapt the much larger eyelets