I thought when increasing the engine size on the logbook you don't need any inspection or signatures only your own Just fill in the form and send it to Shannon.
They don't look a gift horse in the mouth when it's bringing in more tax money.

My experience of this is from a few years ago but i haven't heard of it changing since.

If I were you I would leave it to the next owner to do as the cheap tax will be an advantage to some and it's easier to change up cc than down