Why do you not ask the question in the newsgroup news:comp.dcom.cabling ?
One problem in using Cat6 cable in retrofit installations is that Cat6 is usually thicker than Cat5e. If you are pulling the cable into an old conduit, working with Cat5e may be easier.
We have pulled new network cabling into 20 mm (outer diameter) plastic pipe conduits in Finnish appartement blocks, along side the old 2-pair telephone cable. I have recomended using Cat5e instead of Cat6 because of the smaller diameter of the cable. -[Follow-ups set to c.d.cabling]