Thought I would jump in on this a well since I am in the process of converting my system from 6 to 12 but later down the road in the restoration process.

So right now I have an old 6 volt fan motor and a heater/fan switch that when you turn it the fan goes faster....sort of like your light switch when you twist the knob the dash lamps get brighter.

Anyways I was planning on going ahead and replacing the fan motor with a 12 volt one. Question is will I have to make any changes to the switch itself. The switch is pretty much self contained with the exception of a little electronic component on the outside with two little wires and a spring. Not sure if you call this a resistor or capacitor or what.

Any thoughts on the switch work needed? I have no problem with seeing out switch made for a 12 volt if I have to. Don't want to risk any catastrophic electrical issues
As always this is a great forum with a lot of great advice for those like me trying, step by step to bring this old 6's back to life.