The green-is-up versus green-is-down issue is not unique to Poser. Apparently there are other programs that do not agree with each other on this. This is the reason that programs used to generate normal maps offer some sort of option.

It is easy to change in the Poser material room, using two Color_Math nodes. In matmatic notation, you want:

MAGENTA * normal_map  + GREEN * (WHITE- normal_map)

I don't have Poser with me so cannot make a screen shot. But basically:

Color_Math:Add
Value_1 = RGB 255, 0, 255 and plug into the existing normal_map
Value_2 = RGB 0, 255, 0 and plug into ...

Color_Math:Sub
Value_1 = RGB 255, 255, 255
Value_2 = RGB 255, 255, 255 and plug into the existing normal_map

The Color_Math:Sub is making a negative of the normal_map

The Color_Math:Add is combining the positive red and blue with the negative green.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)