What's the brand of the trailer? If was one carried by someone locally, I'd check to see if there are two versions of the same trailer, one with the heavier axle. Never know, you might get lucky, otherwise.....
Differences I have spotted at a glance with TSC/Lowes/HD trailers.
2000 Lbs
Usually a mesh deck or plywood.
Straight tubing used for tongue
4 lug 12" wheel & tire setup
Axle does not have flange necessary to add brakes
1 7/8" ball coupler usually used (although a 6x10 version might have a 2")
3000 lbs/3500lb axle
2x decking
A-frame tongue
5 lug (usually the popular 5 on 4.5" pattern)
Axle has square flange required to mount brake assemblies
2" ball coupler usually used
Most of the "box store" trailers I've seen with 3500lb axles under them are downrated to 2990 to avoid state regulations for titling and brakes.
Converting a 2000lb trailer to a 3500lb trailer would at the very least entail the axle swap complete with new wheels and tires rated to handle the load, some kind of decking upgrade, reinforcing the tongue or converting to an A-frame design (easy but $$ and a trip to the welder's if you aren't able to DIY) unless the tube used is very thick walled. Then there is the re-registering and titling as a "homemade" trailer if you really want to make it legit. Should you get stopped by DOT (unlikely for homeowners with single axle utility trailers but....) they will go by the nameplate specs unless you have proper documentation to show otherwise.
Most states require the trailer to have it's own brakes for GVWR's of 3000 lbs+ so if you really want to legally use all of that 3500lb axle rating then you'll need new backing/brake assemblies, drums with integrated hubs, possibly new wheel bearings and seals, upgrading to a 6 or 7-way plug, and installing/wiring a brake controller in your vehicle.
Selling the 2K and upgrading is starting to look real attractive eh?
