Modifying a Theme

Hi all,

I just want to get a better understanding of themes. A lot of people seem to say create child themes if you want to modify the parent theme.

If I were to use the twentyfourteen theme, duplicate the files and rename the folder and then modify files, like header.php, footer.php style.css etc… Is there a downside to customising themes this way? Obviously I know I won’t be able to update it, but will that affect the theme with newer versions of WordPress?

Cheers,
Al.