Hi There @PHTGRPHR
Thanks for writing in! You will need a child theme, when you want to integrate a particular customization code to be inserted into your X theme's template file. In other words, if you want to edit X theme template files, we recommend to setup a child theme first so that your customizations will be remain separately in your child theme.
If you do your changes directly to your main X theme (parent X theme), it will be erased when you update your X theme.
So in your case, if you haven't done any customizations to your theme's template file, you can setup your child theme anytime (before updating the theme or after updating the theme). If you have done any template edits, you need to setup a child theme and move those edited files in to your child theme first, before updating your X theme.
Please follow our child theme setup guide on our knowledge base section here (https://theme.co/apex/forum/t/setup-how-to-setup-child-themes/57).
Hope that's clear.