Where to put a migration script to switch post information?

Your options are:

1. Paste the code into a plugin
2. Paste the code into a mu-plugin file
3. Paste the code into a theme file like functions.php

I manage this kind of thing with a plugin that builds a simple interface– basically an “execute” button. I alter the code that executes as needed. That seems to give me the most control. The code only executes when I navigate to the right screen and run it intentionally.

Pasting into functions.php, or similar, and then quickly deleting the code is not a reliable way to run code once. That code will run as many times as your site has visitors before you can delete it again.

You could save an option to the database on first run to prevent a second fire, but that seems like overkill for this kind of “maintenance” code.