It is only possible to edit the VAT type of an account if there are no transactions associated with that account.
In order to change the VAT status follow the steps below.
Select the supplier or customer account you would like to change
Select Edit
Select the VAT type
Select the new VAT type from the drop-down list.
Select Save
If no drop-down list is shown when you select the VAT type then it is not possible to edit the VAT type.