I'l try to give a quick workflow runthrough.

Your goal is to make Vicki 9.9's breasts look less realistic. You take the base figure into a modeling program and do a great job of it. Save your work as a new .obj with a readily identifiable name back to the same folder. Then open the .cr2 file in a text editor and change the two references to which .obj file/geometry file is used by the character to the new name.

The two key files to making a character go are the .obj and .cr2 file. If you save the morphs to an external file, you also have the .pmd file, but just leave that alone.