Upgrading Prestashop from version 1.3.1.1 to 1.4.x

[nac78]

I've been running my store since last year with the 1.3.1.1 version and it has been working great.
The simple question is, what should I do if I have a modified this files

Root
- contact-form.php
- order.php
- cart.php

Classes
- Adress.php
- Links.php
- Tools.php
- Product.php

Config
- settings.inc.php
- smarty.config.inc.php

Should I also redesign my theme to fit the new 1.4.x version?

Please tell me your opinions, I will wait to update until I get a clear image on this.