I've had brow bone reduction done via coronal incision. How safe is it to do revision via existing scar.

  Forehead contouring revision can be done several ways but using the old scar may not be possible if it is too high. For more than 35 years I have used the Irregular Trichophytic Forehead Incision to contour the forehead and lower the hairline if too high.

If you had a bicoronal incision, it can be used easily again to expose the entire forhead and do what is needed.

Your bone was contoured which I take it that the boney bossing was shaved.The question is what is bothering you now. If more bone needs to be shaved then an x-ray is needed to make sure how close to the frontal sinus one is . and if more bone can be shaved safely.

If there are irrigularities then you may need an Alloderm graft to smooth these irrigularitoes.