I have a Mid 2011 Macbook Air running on Lion. I plan on purchasing a retina pro soon for myself and then handing my Macbook Air to my sister and then she'll hand off her nonretina Pro to my parents. I want to erase all the contents from my Air so that it's as if it's fresh out of the box for her. I don't want to have to upgrade the OS though as I don't want any complications with the laptop slowing down. Is there a way to start fresh and keep Lion?