That is a technique which involves the use of chakra. Again that is a different subject, however the only page that support a possible change in the mirros's shape is that we have seen it in a circle shape. Later we saw it in a semi circle shape. A circle and a semi circle are two different shapes, but this could also be argued as a size manipulation because it hasnt reached its intended size to become a circle (if that makes sense)
I did however it needed clarification and support from sources instead of people just coming out and saying it without any source to back them up