Your 3,500 pound axles probably have 2 3/8ths" tubing for the axle housing. The 5,000-pound axles have 3" round wall tubing, so that would definitely be a stronger axle assembly.
Upgrading the axles from 3,500 pounds capacity to 5,000 or 5,200 pound capacity is a simple mechanical R&R, but you probably cannot simply replace just the axle housing (tubing). You'll need to replace the entire axle assembly, including hubs and brakes. And with the new hubs, the old wheels probably won't fit, so you'll need new wheels too - and that might mean new tires.
If your trailer has Al-ko axles, then a good source for new ones is
5,200 lb. Standard Spring Trailer Axle
About $350 per axle with brakes.
If your trailer has Dexter axles, they cost more - about $466 each with brakes. Here is an example of a straight axle with a 84" hub face.
5.2K ASSY 84-70 655 ELEC EZ OV
To order new axles, you must know the length of the axle, called the hub face measurement, or the distance between the hub faces (base of lugbolts) of the axle. The example of the Dexter axle above with 84" hub face is typical for normal-width TTs. But you must measure to be sure.
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