I have a megahood, and for some reason I decided to use the Shopping District version of the Belladonna Cove subhood, and I'm wondering if it's safe to flag it as a Downtown? I know that it is safe to swap hidden subhoods to shopping districts to edit them, but that's a temporary measure tafter which you set them back to hidden, and there are no households living on lots in hidden hoods... Is what I want to do safe?

Theoretically, yes. But there's more to it. These threads might help:
https://modthesims.info/showthread.php?t=502306
https://modthesims.info/showthread.php?t=553111
https://modthesims.info/showthread.php?t=655536