How do you change the valve spring on an installed head for a softer spring? I've heard of people using compressed air in the cylinder but I've never done it myself.

I used to do this stuff using a scrap head. Just cut it down so you have 1 cylinder's worth left and then it's easy to fit and remove

 

There setup springs,there real light and go on with the keepers/retainer.

 

I just use compressed air to hold the valves in place and a custom spring compressor the remove the springs. I don't even have the piston at TDC.100 psi holds the valves very good