I did look into message id numbers, yes you can change them and play another message. If you do, you risk orphaned audio files. In my test, I did reference an audio file in use elsewhere and there may be logic to not change the original.
If I get the time time, will set up a template with canned audio files and see what happens when changing a daughter flow. will need to make at least 2 from it to see how this works. FWIW, I use the templates to create most auto attendant callflows. I also use it to back up ones prior to a change in case I need to go back.
My basic template starts with a caller id filter then override. Override on...override menu, override off,,,holiday schedule. off hours holiday menu...on hours day/night schedule.off hours night menu...on hours day menu.
Using templates makes most actions ids number the same. This does help to say what greeting may need changing