It doesn't NEED to be, but it will read negative rather than positive unless the wires are swapped around.
As long as you know it moves to the left rather than right when it's charging ......All is good.
I've actually left one like that on purpose.
You will need to reverse the polarity as the 12v systems are most always negative ground as opposed to 6v systems which are most always positive ground. There is a Sticky post near the top of the Allis forum about changing an Allis C to 12v started by Wizzard which may offer you some tips.
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