I have a migrated iPhoto library
If you are no longer using iPhoto, you can copy the iPhoto Library.migratedphotolibrary to an external drive, to save it as a backup of the originals in your Photos Library.
You will probably not gain much storage by deleting the iPhoto Library. When Photos migrates an iPhoto library to Photos, the new Photos Library and the old iPhoto Library will share the storage for the original image files by hard links. Since the the originals are identical in both libraries, they will use the sam disk blocks. But you can delete the iPhoto Library and the Photos Library will not be affected. The linked files will only be deleted from your drive, when the photo will also be deleted from your Photos Library. As long as one library is using the image file, the storage for it will not be released, see:
Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support