The safest course is to leave the fabric untouched, and move the strap/buckle as described. There is a limit to how much you can take up that way---you might start to feel that the buckles are meeting in the back and it throws off the geometry a bit --- but it sounds as though less than an inch is needed?

If someone does this for you, they may think initially that each side must be moved half the needed amount. Since it's a wrap-around, BOTH sides need to be moved the full amount. So if the waist needs to be a centimeter smaller, then both sides move by a centimeter.

Finally, if you spring for a copy of The Book you'll see how to do it yourself; and it will give you an appreciation for how this nice garment is put together!