Oh boy, this is a popular topic and a difficult one to answer with alot of agreement. I would search the archives for this. You should find a ton of info including diagrams.
Basically as I understand it.
you do not have to change cables as 6 volt uses big cables already.
You can change it to neg. ground if you wish, but you must change the wires on the ammeter and the coil.
you must install a resistor somewhere around the points(this is where the argument starts)
If it is a magneto you have to only change the generator. Rebuild it for 12 Volts or install an alternator, which you must get creative to do.(new brackets, belts and wiring changes.)
change the headlight bulbs.
The starter will work just fine but will heat up faster.
I don't see any reason to change over. Clean all connectins real good. Replace/recharge a weak battery. perform a good tune up and all should be just fine. The tractors were made to start on the 6 volt and even my 52 started before my engine rebuild and it ran as rough as they come. It burned more oil than gas.
Its up to you. decide if you want to restore to use, or restore to show off. Using a 12 volt with accessories and such is handy. Showing off a 6 volt positive ground system and how cool nostalgia is is fun for me. And it starts on the first crank.