Unless the rules have changed for this recently....
If it is going to wear the new barrel you need a registration certificate amendment to reflect the shorter barrel length (new reg certificate will say less than 470mm) to the CFC, don't need to re-register.
No the new one cancels the previous one, as long as it lists the shortest barrel that is all they want, going longer is no problem....at least that is what I was told when I did mine about two years ago
Cheers



























