If, as is most likely your van is fitted with an ALKO chassis then you need to crawl under and look for a plate in the middle of the axle. This gives the maximum weight you can have, and if it is the same as what you currently have, then you are out of luck. If it is higher, then yes, you can replate to that weight.
Do not bother asking ALKO if you can get the axle upgraded as they do not want to know. Why? because I tried,. They will give you lots of excuses that the elements have to be replaced and it is too costly and difficult. Basically, they would prefer you bought another van with one of their chassis. You may even see your chassis under a heavier van, but tough, it won't work.