People come in all kind of shapes., RTW makers can only make there garments in one average shape and hope fore the best.
You have a pretty good idea of the solution. Your tailor will grab a handful of cloth and make some chalk marks. This will show
him how the new seam will be made.
When the alteration is done the thigh circumference may appear too large, but this is a separate problem, and a separate
alteration, for a separate price.