I sent in the appropriate paperwork, including SV Tech report etc to change from 3500 MPM to 3700. I've had everything back, they have changed the revenue weight to 3700 and confirmed that a refund for vehicle tax is on its way to me. I have called and spoken to DVLA, they didn't seem to fully understand why I would want the MPM to be changed and suggested that I apply again with a covering letter. I'm assuming that I am right in thinking that common sense says the 2 should be the same ?