Defining child theme after the site is done

Hello , I just have one question. I’ve missed the installation of the child theme at the beginning of the process and worked on the parent theme. now the site is done and pretty customized and I want to update to the new version but if I do it I will erase my work… so as I understand .. I need to define a child theme… is it ok if I do it now, this step will erase my work? thank you so much for your patience!