Q: library after migration to Photos

That would depend on what you want to delete.

If you are absolutely sure you won't go back to them, you can delete them. That's up to you. The libraries are actually hard linked together, so photos aren't actually duplicated and taking up additional space. You should at least back them up to an external drive just in case.

But, you shouldn't have any ill affects with deleting them. Just don't delete the Photos Library.

You can move them to the trash, test your Photos application, then delete when you are confident.

You may be surprised - but certainly you can if you want - just be aware that it will make going back very difficult and will save very little space - Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support - you save only some dabs and small overhead files space unless you have deleted a to f photos form Photos - in that case the originals are still on disk uptime all references to them are deleted and deleting the old iphoto and Aperture libraries will save that space

LN