Can I Fix Photos That Are Already in My Photo App?
Photos already imported into Apple Photos or a similar library need a different approach. Here is the order that works.
Once photos are inside a managed library, the app owns them. Changing the files on disk underneath it either does nothing or confuses the database, because the app has already recorded its own copy of the date.
The reliable route is to fix the files before importing. If they are already in, export them out to a folder, fix them there, delete the originals from the app, then import the corrected copies.
Some apps let you edit a date by hand for a selection, which is fine for a handful of photos. For thousands, exporting and fixing is faster. Takeout JSON Metadata Fixer handles the folder stage and is free for your first 500 photos.