I sympathize with the OP. I've had the same problem with a recent 638. It was, however, fixable.

Surprised no one suggested an ammo switch. Most of my j-frames will shoot different ammo not only to wildly differing vertical POI, but also wildly differing horizontal POI's. They all have pretty different preferences. Some are zero'd for 158 FBI, some for 125 JHP (golden saber etc), some for 135 Gold Dots.

I might be a butcher, but I twist barrels and file rear sight notches on fixed sight guns all the time. A 1 7/8" J-frame has a very short sight radius and it takes very little adjustment of the sights (twisting of barrel or filing or rear notch) to make a big POI change.

Still, maybe the easiest solution is to buy another j-frame.