I've changed out cables and casing running through bike frame tubing on many bicycles. No big deal at all. If just replacing the cables and casing you just push the old cables and casing out with the new cables and casings. If you're installing cables and casing on a new or refurbished frame you just plug all the holes except for the entrance and exit holes for each and blow string through the tubing with an air compressor nozzle and then pull the casing and cable through with that string.