If you have not made changes to any core file of the theme, nothing will be wiped out when you upgrade to pro version of this theme. Changes you have made from Elementor or editor will remain the same. However, some settings of customizer (theme options) will get reset.
Regarding the font size of a blog post content, you can change it using custom CSS. Please go to Appearance >> Customize >> Additional CSS and add following CSS
We have resent you the purchase receipt. Please check it. You can find your download files and purchase history here
You need to download Blog Kit Pro theme from there and install it like you installed a free version of the theme. Please go to Appearance >> Themes and click Add New. Upload blog-kit-pro.zip, install it and activate it.
To enable license, please go to Appearance >> Theme License, enter your license key and save changes. After that, you can see the activation button, please activate it.