Reporting from R&D.
If you don't have a kit which came per default with the injection molded (black) parts, then the dual-extrusion kit will fit fine. You DO have to use the old top plate then, this will fit fine, but you'll have slightly less room for the cables. (This is not in the assembly instructions)
For example, this my own printer rigged for dual extrusion: http://daid.eu/~daid/IMG_20120514_235612.jpg (with still a V1 hotend, but a top plate for a V2 hotend without injection parts)
You DO need the V2 hotend, if you still have a V1 hotend you will need to upgrade. Else it won't fit.