The original design was marginal. The retaining 'clip' was a rotation 90 degrees to the axle shaft. This would allow the clip to disengage when the rack was rolled in/out. so when the 'clip' wore sufficiently, the clip would release during the movement and allow the wheel to move on the axle and fall off. nothing like a self releasing design. I guess the mfgrs testing didn't do enough cycles to reveal this flaw (or they didn't care). the replacement axle studs have the retention 'clip' in line with the shaft and also pawls on both side of the bar. this should prevent induced wear each time the rack is rolled.
The old wheels were obviously easy to remove. the insertion of the axle clip into the wheel was literally a 'snap' as was installing the wheel assembly on the rack. install time 5 minutes. This couldn't have been much easier unless the 'fix-it farrie' would have done it...
[an 8 pack of just the axle shafts is all that is really needed for this fix as the wheels were still functional and didn't really need to be replaced]