I have 4" and 8" stewmac 12° sanding block as well as a 12° fret caul to check the radius.
Yes, that's almost what I meant by sanding on the center. However to go from 9.5 to 12 I've removed a good quantity of wood from the center.
I gone through my lower 12 fret inlay by sanding too much... I see a brown point in the center and a brown line on the bottom.
It still need a bit of sanding, probably with a softer sandpaper, to be right on 12°
Why buy another neck? I would still be stuck a 9.5° radius and don't have 250$ to spend on another neck.