Yeah, the GS8 is different than a 9. Not just the wiring.
The pickguard is larger to accommodate the extra switches.
The pickguard has a longer lower 'horn' and a point behind the bridge to balance the design aesthetic.
The body has a different, larger routing under the pickguard to accommodate the switches.
All this for just a few dollars more!
BUT WAIT, THERE'S MORE!The neck has a V-shape carved on the back.
The body has a relief carved on the back for comfort.
We did all this to get you to buy up. Years later I heard the term, 'Up Selling'.
It's funny, In my Engineering job now I always up sell and I'm good at it.
And yes, the Master phase & Series/Parallel switching are magic. I think even after I post a schematic, most people will still be lost! Well, or not care...
If you do want a GS9, please sell your GS8 and buy a GS9. IMO, Trying to upgrade your original would fail and you'd ruin it's value.
Chris