I think the confusion is that the DVLA leave it up to you to recognise that you need a different form and a medical report post-70 if you want to retain any C or D class on your licence. If you just apply using the "normal" form these sections are removed:
If you passed your car driving test before 1 January 1997 you would have been given the implied entitlements for vehicle categories C1 (medium sized vehicle that weighs between 3500kg and 7500kg) and D1 (to drive a minibus but not as part of carrying passengers).
If you still require any of these entitlements you won’t be able to make an online application. You must make an application using your D46P renewal form or a D2 application form accompanied by a D4 medical report, which must be filled in by a doctor.
However, if you no longer want these entitlements you can continue to apply online just to renew your car entitlement.
Full details at:
http://www.direct.gov.uk/en/Motoring/Motoringtransactions/BeforeyouapplyA/DG_172842