Hi @benluoma
Why not write a custom function that you run only once? Think of it as a ‘migration’ function.
In this function you could query all posts, and loop over them. For each post, you can load in the flexible content field value, and append the post_content if not already added (somewhat similar to your above code).
This would mean that you only need to run the function once, then comment it out and all your posts should be updated!
Thanks
E
