Prestashop Upgrade

I'm running an online store build using Prestashop version 1.3.1 I did a lot of modification on the themes as well as functions and modules. And i need this to be upgraded to the latest Prestashop 1.4.8 which no longer requirement manual update in the future. All the current functions and modules will have to be compatible and bug-free as we will be running it for live store. I will be copying all the content and database to test server where you will be working on. And from there it will be 100% tested first before mirrored to the live server.