During the maneuvers of '41 many of the national guard units were still wearing surplus uniform coats, so as as has been pointed out WWI could be used for NG, but the regulars uniform had indeed changed.
Of course one prewar soldier told me that the '41 maneuvers was the first time in his time in the army Corp of Engineers he had worn a uniform in the field, most of the time they wore cheap jeans and jean jackets or CCC furnished clothing. He even mentioned being mistaken for a chain gang at one point, so civilians might work too.