Child theme

hi, I have almost finished a website with the free Catch Box theme. Love the theme. . However.... I would like to upgrade to the catch box pro. But, stupid me.... I haven't created a child theme of the free Catch box. Thus... If I am going to use the Catch Box Pro everything will be overwritten.. I know, stupid. Can you please suggest me how I easily can create a child theme after which I will buy the catch box pro? rgds and thanks in advance Iris