The only time I tried to notify the CFC of a caliber change,they insisted I should take the rifle to a verifier. My response was to tell them that I had given them the information and they could do whatever they wanted with it. Guy on the other end seemed a little miffed but, Oh, well!
The truth is, all of my rifles are likely to change caliber at any time for any reason so I just can't be bothered with it. In addition, on none of my certificates is caliber mentioned so I don't see the point. Regards, Bill.