changing parent theme into child theme

That is 100% the approach you should take. You can even set up the child theme with a folder your-child-theme-name/ and then a style.css which uses the parent as the template:

template: your-parent-theme-folder

Then, you can either import the styles into this style.css (@import url('../parent-theme-folder/style.css')) or start from scratch as needed.

You’ll also be able to pull in files you want to edit (like header.php or footer.php).

Additional reading: