Can I change my 04 defender tax class?

My Series was registered as "truck" and I wanted to change it to "camper" as I had converted it. DVLA were helpful but insistent that they could only make changes "to tax classes now in use for new registrations". It seems they have reduced the number of tax classes and so, even if you know of similar vehicles with a class, if its no longer used you can't have it. An example would be "estate". For Land Rovers their preferred class is "4x4 utility" and they seem to want to get all Land Rovers into that. As it was better than "truck" I was happy and the insurers who had kicked the whole issue off were happy too.