I have used the Realistic Human 100 pack since CC1/2, and I have used it for a dark-skinned character while not noticing a major seam as we see now.
I am currently in the process of updating my characters to the latest shaders and after conversion this same character has a noticeable seam.
So I went back and recreated the character the way I made her originally.
I start out with a CC1 character with Traditional shader and then apply the CC1 version of the head. To improve the texture quality I set the size of Body and Head textures to 2048 x 2048 using the Appearance Editor (which works for CC1 characters).
I then convert the shader to PBR. The matching result is much better and there is no seam.
I then converted the character to a CC3, which will also apply the Digital Human Shader (it needs to done separately for the head). The result is still good:
Finally, applied the CC3 version of the head directly to a CC3 character. Now there is clearly a seam:
I also tried my round-about method with the M_Y_003 male character, who has darker skin, and in that case a slight seam is noticeable, but it is still much better than with direct application of the head. If I have some energy I will write something up for FT.