Unlike some EU countries, it's not illegal in the UK to change or modify a vehicle or trailer outside the scope of it's Type Approval PROVIDED that it remains roadworthy. - although in the case of an axle change on a caravan, the maximum authorised weight can't be altered except by the manufacturer or a chartered engineer. So it's not a matter of Type Approval but Construction & Use Regulations.I am guessing a bit here, but the refusal to uprate the axle may be to do with it taking the specs out of those that the van was type approved for and therefore making it illegal on the road.
However. A member on here reports that they have updated their axle on a couple of vans but I don’t think they explained how they covered the roadworthiness on paper.
Don‘t knock me down, I have no expertise in this area.