Add Fields to Post OR Change Post Type of Existing Post

Site URL: https://www.americanlamb.com/

I am coming from a WordPress/PHP background so forgive me if my terminology is not 100% correct.

We are in the process of adding some custom fields via the instructions here.  So far, so good.  I have created a Recipe Custom Type and the fields are appearing in a tab as I'd like (see Screenshot below).  However, our site has hundreds of existing posts and we need to take advantage of these new fields to update them.  What I am unable to determine is how I can either:

1. How can I change existing posts to utilize the new Recipe Custom Post Type without re-entering all of the post content
2. Add custom fields to the default Post Type.

Any guidance is very much appreciated.