Creating child theme from customised parent

Hi there, I have been preparing the launch of a new website using the Adventurous template, and was recently prompted to updgade to version 1.5. Of course, this came with the following statement: "Any customisations you have made to theme files will be lost. Please consider using child themes for modifications." I now understand that it would be advisable to create a child theme. However, I'm a novice user of WP and have already made numerous custom (cosmetic) changes to the parent folder (menus, headers, content, etc) using the tools under the Appearance tab. I would greatly appreciate it if somebody could explain the ramifications of updating my theme... Will I indeed lose all of the work so far, or does this only apply to more in-depth 'customisations'? If so, how easy is it to create a child theme based on my modified parent theme? Are there any step-by-step guides available? Would it be easier to simply take screengrabs/print out the current format and start from scratch? Many thanks for your time.