The functions.php, style.css, home.php, page_portfolio.php, and they’ve added a “lib” folder now which contains the prettyphoto code (IF you’re using that).
If I were you, I’d try doing that all on a test site before attempting to do this on your main site…..just in case you make a mistake somewhere and it takes down the entire site. I’ll be honest, it’s not easy if you’ve made a lot of changes, but if you haven’t then it shouldn’t be that bad….as long as you know some of the basics of PHP, CSS, HTML etc.