For what it is worth, I upgraded my 2500 lb axles to 3500. The dealer did the work -- not me! This was done at the same time that they lifted the trailer, via an "axle flip."
My goal was to make my trailer a little more rugged -- we do a lot of off-pavement towing at slow speeds on very rough forest roads. I figured that the impact of the trailer on rocks and potholes would put a lot of stress on the axles.
I'm not sure that the upgrade was necessary, but it gave me a little extra peace of mind.
2012 Fun Finder X-139 "Boondock Style" (axle-flipped and extra insulation)
2013 Toyota Tacoma Off-Road (semi-beefy tires and components)