Its worth mentioning that not every caravan can have its MTPLM upgraded. There are various factors that need to be satisfied. Firstly just because the axle fitted to a caravan may be rated higher than the current MTPLM, does not mean the caravan can be upgraded, the design and structural strength of the body work may be a limiting factor.
Some manufacturers may not offer any upgrades att all - its a matter of company policy.
Some manufacturers will only offer an upgrade to the first customer or on caravans not exceeding a particular age.
And as the OP has established sometimes the available upgrade is barely worth it.