I actually wasn't that hard once I figured out the procedure which included cutting the side plate on the frame to open it up to accept the new axle which was about 3/4 wider. I also had to add some 3/8" spacers and I made them extra long to distribute the load to the frame better.
The first one took about 9 hours all together. Once I figured out the game plan it took me four hours on the second one. It is absolutely critical to have a 3 ton jack as it is just way to heavy to hold up there yourself.
I also fabbed a template to guide me where to cut axle.
Note thin line in how much I had to relieve the side plate to fit the new heavier axles. I figured Airstream had done a superb job of drilling the side holes in the proper place and I sure did not want to elongate or relocate with new hole or elongated hole on trailer. Thusly I made up a template from the new axle and clamped it in place and scribed the line exactly where it needed to be.
Once I cut out the U shape I was good to go.
I figure that had I had a helper it would have gone faster. As it was I had my best buddy to keep me company.
He is gone now and I miss him so much.