Recently the aspect of my camera changed, previously I had a straight rear view with no parts of the car showing. I now have the curve either side and above the number plate and the very top of the number plate. Is this a usual occurrence and can I adjust it?

 

You can check to make sure your camera tabs are firmly set in the socket though it's highly unlikely to be the issue.
I have read on various website forums where some owners of older models (not sure what year you have) have claimed software updates changed camera view to more of a fisheye.
I have found nothing from Mazda that confirms that.
Some of the Mazda techs here can address that when they read this.