I have no experience with factory wiring, but I know what it takes to go from 12v to 24v.

Let's say your current system is a bow-mounted motor, no other devices on this circuit, one stern-mounted 12v battery, and 2 wires in between. If you have the space to locate a second 12v battery near the current one, all you should need is a jumper. Disconnect one wire from your current TM battery and hook it to the same pole of the new battery. Then hook the jumper between the disconnected poles of the two batteries. You will now have 24v at the hookup point on the bow of your boat. Obviously, you can't switch a single pair of wires from 12v to 24v if your finder or any other 12v device is on the same circuit. I have a heavy pair of dedicated wires from my 2 batteries in the stern to the TM up front and a lighter set of wires hooking my finder to the single 12v starter battery.

If your existing wires are heavy enough to handle a 12v TM, then they should do even better with 24v. Nevertheless, it's a good idea to use a meter to test for voltage drop. If the resistance of the wire causes too much drop, go for heavier wire. Remember--voltage drop shortens fishing time.