Basicly, to have a 223 rebarrelled to 17, you'd first have to apply for a 'one for one' variation ie : replacing one 223 cal with one 17 cal. You'd then have to wait till your ammended FAC came back before you could collect the rebarrelled rifle, even though the serial No of the rifle, and entered on the certificate would obviously remain the same.
In the case of interchangeable barrels of different calibres, you'd have to apply for such and have them entered on you certificate. You'd also apply to be allowed to purchase/aquire and have in your possession the ammunition for the two differe