If you'd like to have your original photo readily available to re-crop (or fix and enhance in any other way), the easiest way is to always choose As a copy (located near the lower right corner of every Fix and enhance page). If you choose As a replacement, your original will still be available, but it will not be visible in your album. To view it, return to Fix & enhance and click Revert to original photo, under the Fix tab. This will undo the entire history of edits on this photo.
NOTE: If you delete your original photo, it will not affect your cropped picture, but you will no longer be able to re-crop or fix the original.