My understanding of the rules:
Is there a way to prove what was done when? You may well need a lot of evidence because the DVLA will start from the point of view that if they're only being told today, it was done today. Which means todays rules apply, which means a whole world of pain.
The second thing that might be a problem is that if it was done a while ago it retains its current registration (rather than a Q etc) because it still had enough points under the 8 point rule. Over time things get changed or replaced and it's easy to lose more points to the point where it should be re-registered. Again under today's rules.
Is there any clear guidance on that second point? Eg my Minus was correctly registered as a kit car with the DVLA and retained it's registration, I can't work out what I can and can't change now without having to go through the IVA. Realistically the only thing that would get picked up is an engine/subframe swap, but I don't know if that would be enough to trigger an IVA on my car, where it wouldn't on a steel Mini.