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The first thing is you always need to make a backup of your website before performing any upgrade or migration if you’ve done that.
If you’ve made changes to the theme files directly, without using a child theme or a plugin, those will be lost.
However if you’ve made changes from your customizer, using Customizer Export/Import plugin should work fine to import the changes back.
Had you succesfully imported the changes in the customizer? What steps had you followed?
Let us know.