1. You could - but it is always a good idea to have backups of your books. You can either keep the originals or backup your Calibre library to achieve this.
2. If you change author and/or title then Calibre will rename files within its library structure to reflect your change. However this should not really concern you if you stick to using Calibre as the way to access your stored books.
EDIT: Looks like Kiwidude beat me to it
At least our answers are consistent
