Most cables had "spare pairs" in case problems occurred. In a hundred pair cable, there'd be 10 unused for example, and if a short / open developed somewhere then you'd use a spare pair. I remember there was a procedure for replacing a cable in a form (the tied set of cables), but not something I ever did, they were in the process of phasing out lacing when I started and I only did it for a few years, there were these new things called "microprocessors" which seemed interesting.