Generally, mechanical-only re-arching (typical "spring shop" process) works initially, but the springs have "memory" and will eventually go back to their original (sagged) position. Correct (and permanent) re-arching requires heating to cherry-red, re-forming, oil quenching, annealing, and shot-peening; Eaton Detroit Spring can do this, but it isn't cheap - it's essentially the same process they use to make new springs.