The short answer is YES, The long answer is...
The purpose of the Child Themify plugin is to prevent this from happening. Here is how it works. When WordPress(yoursite) runs,
1) Basic WordPress uses it's own style.css
2) An activated Theme brings in it's changes to your WordPress site by using it's own Theme/style.css to overwrite the style created by the Basic WordPress/style.css.
3)Child Themify creates it's own folder 'YourChildTheme' in the WordPress/wp-content/Themes folder with its' own style.css. When YourChildTheme is activated in Dashboard Themes, The WordPress(yourstite)/wp-content/Themes/Theme style'css is in turn modified (overwritten) by your WordPress(yoursite)/wp-content/Themes/YourChildTheme/style.css.
In that way, when you update the Theme, your own YourChildTheme style.css is not replaced by the Theme update. A child theme named YourChildTheme created by Child Themify has it's own style.css style sheet located in the Child Theme folder created when you click on .Create Child Theme;. Initially YourChildTheme/style.css contains nothing but
Theme Name: MyCustomizr
Description: A child theme of Customizr
So, any stylesheet changes you make to your WordPresssite should actually be in this YourChildTheme/style.css, NOT your Theme/style.css. That way, when you update the Theme, your changes are not lost.
Regardless, You should always keep a copy of your changes in a file, maybe called something like 'MyChangesToStyle.css' so if something drastic does happen, you can easily restore all of your changes by renaming this file to style.css in the YourChildTheme folder.
Additionally, your next question may be:
How do I add a Dashboard/Theme screenshoot for the childtheme?
per the help files:
1) Create a screenshot and call it "screenshot.png"
(I used MS Paint but any software for creating/editing images will do.)
2) Now upload that to the root of your child theme's folder. In other words, put it inside of the folder that holds the files for your child theme, but not inside any of the folders within that folder.
3) It should be picked up automatically and show in your admin area after you do this. If you don't see it, you may have to clear your cache.
I hope this helps.