If the plan is to install all new wire, then it doesn't really matter too much where the old stuff is. Just cut a new trench and lay new wire. Conduit would be even better.
Is there a possibility that whatever is growing could later grow tall enough to encroach on overhead wires?
I assume by your question that the current wiring will remain energized until the replacement wiring is installed and ready to switch over, correct? If the buried wires really do have damaged insulation, I don't think there's much you can do to mitigate the power leakage while they're buried other than to take the wire out of service. Other than that, I would try to keep the ground bone dry. Obviously, you can't control the rain, but I'd ensure there's no landscape irrigation in the area of the buried wires. I also wouldn't want to dig anywhere near the wires unless they were first de-energized.