Extracting photos out of very old iPhoto Library to import directly into Apple Photos on El Capitan I have many many photos on a very old version of iPhoto (pre 9.0) that's proving a pain to upgrade into Apple Photos on El Capitan.

Question is - can I simply copy the "Originals" and "Modified" folders that are inside the iPhoto Library out into the Pictures directory itself, and then import these into Apple Photos directly?

EDIT

Also - if I do this, will the copy necessarily take up twice the space in my hard disk, or will disk level de-duplication help to reduce the disk used? If it takes up twice the space this may not work as the photos take up more space than I have free at the moment.
You can, but you'll lose metadata such as albums and you might end up with duplicated photos if you import both the modified and the original.

Instead, you can use the iPhoto Library Upgrader to prepare the library for import into Photos directly. This Apple tool takes your pre-'09 library and allows it to be imported into Photos. For more information on how to use the iPhoto Library Upgrader, see Using the iPhoto Library Upgrader tool.

Pre-'09 is v7.x and earlier. v8.x and later can be imported directly with Photos without this tool.