upping gvw

[Darlingtonms]

While talking with a friend of mine today, he asked a question I had no answer for. I was explaining that I wanted to build an aluminium bed for my toterhome to keep the weight down on the rear axle. The truck is 25500# gvw with a 17500# rear axle. He asked me why not put a heavier axle under the rear and up the gvw of the truck, providing that the frame was rated for the higher gvw. I don't think you can upgrade a gvw on a truck, although you can use heavier parts and have the truck derated to a lower gvw from the factory. Anyone had any experience with this, or know the answer?

thanks in advance
jimmy