When you change the theme your ACF fields are still there in the admin as well as their values on pages, posts etc.. However the new theme will most likely not display the fields since there’s no code for them in the theme-files.
You wont have to do anything to ACF but rather add your fields to the theme-files in the way they where added to your previous theme. If you do not have the skills to do so you’ll either have to stick to your old theme or hire someone to fix the new one!
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users