Negate the numeric value in the displacement channel - that will reverse the sense of the disp map.

Basically the normals (and your displacements) were all backwards. You flipped the normals, but the displacements were still specified in the direction before you flipped them. If you set a disp-map up with a value of, say .02, then -.02 will do the same amount, but in the opposite direction.

A lot of people think that displacement can only be "outward" but that's because you can't make an image with "negative" numbers. A negative displacemnt is "inward", and you can get these by either doing math on an image (for example, mutiply * -1 as I'm discussing or you can subtract .5 - thus 0 to 1 becomes -.5 to .5) or you can generate displacements entirely without images, using nodes alone.

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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)