When calling the update method, the HTML generated in $post->text()->kt() is converted into Kirby Blocks of type header, image, text, list… automatically. If you want to keep everything in a single field, that’s fine.
In my case, what I find is that the image blocks were transformed to images with absolute paths in which you could not work well for the migration and I had to detect the image blocks and modify them so that they were inside the Kirby panel.
I leave you a Gist that I have published for you to see, I hope it sheds a little light on you.