If they didn’t staple the existing runs, pretty easy, just disconnect one end, tie some nylon pullstring to it, and pull the opposite side through until you reach the string. Then simply tie it to the new cable and reverse the process.

If they stapled the existing runs, then damn near impossible without ripping some walls open.

E. I would strongly advise against tying the cables directly together. You risk damaging both cables and the knot is much much larger that way, meaning you’re likely to get stuck somewhere. Also I can’t describe how defeating it is to wrestle with a cable for two hours only to realize it was damaged in the process when you finally get it though.