Ozie if you upgrade to the new version 1.2.4 you shouldn’t need to update to the new child theme. Leave that one alone and upload the preferential theme only, “not the child theme”. This will leave your current Child CSS alone.
If you have changed any of the PHP files then yes they will be overwritten with this update.
If this has been your concern maybe this will be helpful to you?
Always be safe with a backup of your current Preferrential and Preferrential Child theme first just in case of any problems you can revert back to that one.