It will be a lot of work, if you really want to do it, and it is a risky operation, I really cannot recommend to do it ...
Back up your library, before you read on.
- First of all, to make the jpegs the original master image files instead of the raw images select the images in the Browser and use the command "Photos > Use Jpeg as Original".
- Then turn your images into referenced images and move the originals to a folder outside your Aperture Library by using the command "File > Relocate Originals" and selecting to move the files, not to copy them. Pick a folder where you want tthe images to stay permanently; you should not try to move them back after deleting the raws - that will result in error messages.
- Then select an image in the Browser and use the command: "File > Show in Finder". In the Folder that opens, you can delete the raw files for each image.
- Test, if you can still work with the image and edit it, after removing the raw. If there should be any problems, put the raw back and forget about it.
- But if all is fine, remove more raws, as you see fit.
- I'd keep the raws for the images that are most important to you, however.
You are welcome, dt.
You might consider to turn (part of) your library into a referenced library instead:
Move the masters/original images of the images in your older projects to an external volume ( "File > Relocate Originals") and make the Aperture library smaller this way. If you ensure that all images have high quality previews, you can browse and tag all images, rate them, share them in the Media Browser, create albums, even if the external drive with the original images is not connected. You will only need to connect it, if you want to adjust or export the images, that are referencing the originals on the external drive. Keep only the originals of the projects you are currently working on managed, or all your most important images.
To render high quality previews use the Preview pane in the Preferences panel and set the size of the previews at least to the size of your screen. So you will have the previews to browse all of your library on the internal disk, and the referenced originals on the external disk, but only need to connect them if you want to edit older images.