OSCommerce VAT Exempt

We are building a site in oscommerce which aready has a VAT modification as it is selling disability products which are VAT exempt if you are eligible. This modification is already in and working but at the moment prices are shown excluding vat and when you go to the checkout you have to say if you are eligible for VAT exemption and if the answer is No then the VAT is added onto the price.

This modification needs turning the other way round so that all prices are shown including VAT and as you go through to checkout if you are eligible for vat exemption then the VAT is taken off the price.