Has anyone flipped their overload spring? Is the job hard? Can it be done without air tools? Can you reuse the old hardware? ie u bolts and nuts? How much should you torque the nuts?

When U-Bolts are properly torqued to their specification (as should be expected when installed at the factory), the bolts stretch, especially in the thread area where the root diameter is less than the rod diameter of the bolt.

Steel, once stretched, has reached plastic deformation. In essence, it has yielded, as the torqued nut fixtures the elongation of the bolt.

Among the many properties that steel is rated for, there is yeild strength and there is ultimate tensile strength. There is a lot of forgiveness in steel up till the point where it has yielded. After it has yielded, any addtional loads approaching the point of tensile failure can be sudden and without warning.

To reuse the existing U Bolts, you would be taking U bolts that have already yielded, and then applying addtional static loads on them to acheive the correct torque specification necessary for them to have any retention value in holding your axle to the springs. On top of that static load, they will then be subjected to omnidirectional dynamic loads while driving down the road.

There may not be enough margin between the yield strength and tensile strength to play around with. New Ubolts, on the other hand, have not already been subjected to plastic deformation, so there is a great deal more elasticity and strength margin to work with.

I have no idea why you would want to reverse the overload springs... but to answer the second part of your question I can say with absolute certainty that if you undo the U-bolts to your axle and spring pack, you should recycle them in the iron scrap yard and replace them with new Ubolts and nuts to match.