With my current providers, Sky, it wasn't worth declaring the 16v conversion when I did. I had 3 months left of the policy and they charged me £120, which would have been the same cost if I had declared it at the start of a new contract. It's stupid and I felt ripped off but I had to do it to run everything legally! But they should have charged however many months I had left, so £120 for 12 months, 3 months left should have been £30!