S3 is seemingly costing us a lot. While it has its benefits, I think we could do with moving images from s3 to the server we are running the site from. How difficult/impossible of a task would that be?
Ignore my answer. Andris' is better.
There is no functionality in Craft that I'm aware of, but you should be able to handle it similar to moving a website. The hardest part I think would be updating fields and references within entries.
- Set up your new asset source (whether local or another object store).
- Move all of your files to the new source.
- Go to the settings page in the control panel. First icon under Tools is Update Asset Indexes.
- Add your new source to Asset fields, and go update all of the entries.
- After the entries are updated, you can just delete the old asset source.
There's really no automated way to update fields and entries, but it might be possible with a really clever plugin.