Yes, you can delete iPhoto library after migrating to Photos app. But it won't free up as much as space as it looks like it will and it will prevent you from using that library with iPhoto again. If you migrate an existing iPhoto library to Photos app, Photos creates a new library structure but does not duplicate your photos. Instead, Photos app saves disk space by creating links to the original and preview versions of your photos.
Once you have migrated your iPhoto library to Photos, you can still use your original photo library with iPhoto. If you make any changes in iPhoto, then it will remain only in those applications. So if you create a new iPhoto album, it will appear in their respective apps, not Photos. So don't delete you iPhoto library after migrating to Photos app for OS X.
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