An ATM increase is for increasing the overall load capacity of the van, dependiing on the suspension type it can be easy or expensive, the first thing which can make or break the idea is the load rating of the chassis as that can't be changed, some chassis on smaller vans are only 3 ton, axles, springs can be changed to accomodate an increase in loading ........................
If you want an extra couple of hundred kgs check the load rating of the axles they are often a few hundred KGs above the GTM figure even ATM so all that is needed is the vehicle weighed, checked and a Mod Plate fitted showing new ATM/GTM , new rims and tyres may be required otherwise its new springs/axles....