If you want to change between say different Genesis 3 Female characters, like replacing a V7 withe Eve 7 character. You can just go to the shaping menu, zero out the current shape and apply the shape you want. Many characters also have shaping only presets that you can apply. You then need to change the skin by using the material presets for the skin and whatever makeup you have.

You may need to adjust the pose after changing the character shape, since the pose may not look correct with the new shape. For example, if the character has hands on hips, the hands maybe slightly inside the hip or above the hip depending on the shapes.

If you want to replace a V4 character with a G3F, that is more complicated. Your only choice would be saving the pose (File -> Save as ... -> Pose preset), dressing G3F in the same clothes and trying to apply the saved pose. There are some pose translator scripts, but my experience is even with those you need some adjustments.