It is an interesting suggestion and one I can't imagine the insurance companies offering without a fight. It's true, using your example, if you change your car during the year and it has a lower insurance rating, they will reduce the premium accordingly. I have known premiums change because of speeding points being added or removed from a licence, but have never heard of it being challenged part way through the year.
Work out what it would cost to cancel the policy and re-new with a different provider. Could you make a saving? Many convictions don't alter motoring premiums that much.
I approached several of my insurers last year because under the new LASPO rules, my conviction is spent and although the act was law, it still hasn't been actioned, so they refused to reduce the premium at all. Now it has been delayed from April to November, it means yet another year of inflated prices! Not only that, but the cover is reduced which is a real worry.
Who said life was going to be easy?
Well, who said life was going to be easy?