The 'upgrade' that you referred to is not a true upgrade because the higher MTPLM was there already. It was even plated at the higher figure on the existing statutory plate. A true upgrade always requires a technical change - at least different tyres, more often a new axle.
For manufacturers to cover such true upgrades they need to include them in the type approval documentation as a possible variant. If that is done, there would be no problem in offering different axles or even different chassis options for the same model. Most Continental manufacturers do this, offering their models in 100kg upgrade increments.
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I realised that, as I said, it was a "weight plate" upgrade. "as Axle was already rated at the higher rate".
It's a paper exercise as (I believe) because in theory, you could downgrade the the Weight plate to 1235kg to bring my train weight in line with the 97 licence regs Axle weight still plated at 1350kg.
Confused am I.