Just remember, installing upgraded springs does not increase your trailers weight rating. You can't carry more load just because you increased the springs. For that reason, the load on the axles will not change as a result of a spring upgrade.
When my spring broke I contacted Lippert and they took care of new springs on the one axle but would only replace with the same springs as original. They said they were not allowed to install heavier springs even if I paid for it. I have 4,400 pound axles so the original springs were 2,200 pound. My "on the road" trailer weight is 7,800 pounds. I was 1,000 pounds under spring capacity anyway. When I got home I replaced all four springs with 2,500 pound springs. Now I'm 2,200 pounds under the spring capacity. A little more margin of error.
At the same time I installed a wet bolt kit. Between them and the springs I really improved the handling of the trailer. Not to mention the peace of mind from springs with more reserve and spring shackles with some real strength and stability.