Jan,

I’m afraid other than deleting, there really isn’t an easy way. You could export as JPEG then reimport them into the same project so they’re kept together, but no there’s no easy way to do that.

Seems like it’d be a great feature; kind of a “flatten” command, that reduces an image to a JPEG and tosses out the RAW. Of course I can’t imagine Apple adding that simply because of the risk of someone doing it accidentally.

Of course if you have Previews turned on, and you’re working Referenced, you could simply discard the RAW files and then whenever you want to do anything with the photos in Aperture, just drag them out (you won’t be able to use any Export commands).

Then that does beg the question of “why bother the risk”, when you can buy an external FW or USB drive for really cheap these days, and just move the RAW files there, and stick the drive in a box somewhere just in case.