Hi,
Unfortunately it doesn’t exist any easy workaround, That’s that in any massive theme parent change.
You to check all the changes you made, if you changed functions that use prior to the function declaration the function_exists method (function_exists('function_name')) {
you can copy those functions and paste them inside the functions.php of the child theme.
Other files like the ones in the root directory you just have to move them to child theme directory.
Other files that are deeper in the directory structure and are included in other files, you should also copy the file that call’s the second one so it can call the one in the child theme and not the parent one.
-Rui