17 seater minibus to a camper - tax class question

I’m in the process of converting a 17 seater minibus to a camper van.

I told DVLA that I’d removed 15 seats and have been told I need to change the tax class to a private HGV.

I believe I can change it to a camper van tax class once the conversion has been completed.

Does anybody have experience of having to make a change to an intermediate tax class, which looks like it might be expensive!

I’ve tried to keep this short so there’s information missing. Would love to have a talk with anyone knowledgeable in this matter.

All the best,

Clive