I've swapped stems several times (when the bottom OD is the same), just grind off the weld on the bottom, press out the old stem, press in the new, run a weld around the bottom.
I've also cut stems in the middle, took out some, welded them back together. Believe it or not there is not a lot of stress on the middle of the stem. Most of the stress is at the ends of the stem where the bearings are located.
JohnnyB