Hi a.dewilliencourt,
If you make a child theme then while updating to new version your customized code will not be lost. So there will be no any issue while updating to update version.If you make a direct changes in the code then each and every time while updating to new version you need to customize the code again and again. View this Child Theme link
Unhook function means to remove the core functions using child theme and your customized functions will work on it. If you are unable to create a child theme then you need to hire a developer to fix it.
Thank you!
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