Reclassifying your chassis type will not change your taxation class unfortunately.
As for the LEZ... there are huge threads detailing this subject, but essentially it depends on a combination of chassis type, engine type, number of seats, max load capacity and whatever else TFL invents on the spot. So, no, having a Kombi caravelle or WV2 chassis will not help with the LEZ.
The immediate advantage of having a VW2 chassis is that they came prepared from factory to accept certain types of seats and seat belts. This makes fitting these at a later date much less of a headache. Loads of threads on this too.