I still see a fair amount of existing lead cables and most people are too cheap to automatically replace them.
My standard answer to "can we splice into the existing lead wiring and re-use it?" is as follows:
"It is indeed possible to splice old lead cables, but it's also possible to drill a hole in a potato chip. I'll give you an alternate price for replacing them so that we're covered both ways".
In my experience, as soon as you touch them there's a gigantic probability that they will fail.
