This question has come up in the past check this thread
http://www.mapleleafup.org/forums/sh...highlight=volt . Having said that it all depends on why you would want to change from 6 volt to 12 volt. All three of my trucks are 6 volt. Now as to the electrical system on CMPs many people in the military vehicle hobby are convinced that converting over to a 12 volt system is necessary. But poor performance of starter or lighting circuits is more of a sign of something wrong rather than the difference between 6-12 volts. CMPs almost universally need to be rewired after 60 years. Break down of the insulation corrosion at connections is universal at this point. Another issue that must be recognized is that most CMPs were built with steel wire being used instead of copper, thus voltage drop is significant. I mention this as a vote against using NOS wiring harnesses even when available. Going over to copper wire with modern vinyl insulation and cloth covered with proper color coding is a practical alternative. This can then be covered with the woven loom covering as originally used matching in color and trace pattern. Wire size is critical as are grounds, starting at the battery the main cable to the starter need to be sized correctly a 12-volt battery cable will not carry the amps needed to crank the engine at the intended starting rpm and will rapidly deplete the battery in waste heat. A true 6-volt cable will have a conductor of ½+ diameter.
The generators on Chevy CMPs put out up to 50 amps so recharging the battery is not a problem or running all the lights while driving. Replacing the light bulbs with ones give more candle power is not much of a problem.
Modern 6 volt batteries deliver significant more cranking power than 6 volt batteries of 60 years ago. Thus even cold starts (well below freezing) are general not a problem for CMPs. Having said all of this if you are starting a rebuilt or tight engine for the first time my practice is to use a 12 volt battery connected only to the starter and a 6 volt battery to the rest of system. Don’t over crank the starter and check it to make sure that it is not getting hot.