Give them a choice:

To spend time tracing to figure out where everything goes will cost $x

To rewire everything properly will cost $y.

Make sure $x and $y are pretty close together and put on the sales hat and bridge the gap to go for the rewire. It will save a lot of headaches next time they want to change something and they call you.

Maybe they want to add more lines, maybe they want jacks in different places.. Sell them on solving all their telecomm problems [Linked Image]