You have a couple of potential solutions for transferring your G3 skin to G9:

If your G3 skin is not using the Base Female or Male UV, you can utilize Map Transfer to convert it. You can also consider using products by Cayman or Sickleyield to map the skin to G9. If it's not based on the standard UV, you can use these products directly without the need for conversion.

I happened to rediscover a product I had previously purchased called "Ultimate Unwrap 3D Pro." This tool facilitates conversions from G2 to G8. The process involves saving an OBJ file for both the source and target models, including all the textures, and the program will project the textures accordingly. However, there are some considerations to keep in mind, such as ensuring both objects have the same shape, exposing as much of the mesh as possible (e.g., open mouth, spread fingers and toes), and adjusting the eyes to prevent smearing of textures. While it may require some initial effort to set up, this product provides texture conversion capabilities. You can find a tutorial on their website that explains the process in more detail (look for "Tutorial 3 for Repainting Mesh UVs - Texture Transfer").

The first option is likely the most straightforward and should work well for your needs.