Yes. You can delete your old library after migrating to Photos app for OS X, but don't do it, even if you are tempted to. If you are looking Photos Library in the Finder, you will see all of your original photos and previews. But the photos exist only in one location on your hard drive, even if you have multiple photo libraries.
Here is what Apple has to say about this to people:
"After you migrate your iPhoto or Aperture library to Photos, you might feel tempted to delete your original iPhoto or Aperture library. Because the migrated library takes little additional space, you don't need to delete the original library."
You can easily use your original photo library with iPhoto and Aperture after you have migrated it to Photos app. If you make any changes in iPhoto or Aperture, it will affect only in those applications not in Photos app.
Automate backup & migration of your photos & videos with support for all top services.
Trusted by users in 125+ countries.
“Your pictures are scattered. PicBackMan helps you bring order to your digital memories.”