You are right, sorry...
The 'date taken' only works at import time with the downloader option. That's good if you don't have imported the pictures in a catalog before.
If noboby can help you with already imported pictures in the Elements catalog, I suggest you have a look at various free softwares which can do that. I just tested it successfully with Faststone PhotoResizer.
(Beware that If you are using the organizer, renaming your files with an external software will result in disconnected files.)