Deleting iphoto library after migrating to photos?

I have a photos library of over 700gb and an iphoto library of over 200gb. When I try to open the iphoto library, I get a message saying that "this library has already been migrated to photos."

Does this mean it is safe to delete the old iphoto library? As far as I can tell, all of my photos ARE in the new photos library. I wasn't sure if the photos app needed the old files somehow.

I just moved everything to some external drives and want to be very careful before I reclaim that space on my hard drive.

Thank you