I had my rear's re-arched when I first bought my XJ years and years ago. I don't remember the exact price, but it was over $100 at a spring shop in Sacramento. It didn't last long. I have since read about this non-lasting effect from time to time.
My personal advice would be to purchase a new set of leaf packs. To re-arch, the old pack still has to be removed, so just have a new pack swapped in instead. You can even purchase aftermarket packs with overloads (the flat non-arched, tappered bottom leaf) if you prefer.
Take it for what it is worth.