reimporting photos from the iphoto app to the photos app

Back in 2015, I upgraded my MacOS and got the new photos app. At that time whatever was in my iphotos app was added to the photos app. Unfortunately instead of converting to the new app, I continued to use iphoto for uploading and organizing my photos.

I'm concerned now that eventually a new OS system will come along where the iphoto app will be removed and I will lose a ton of photos. So I am looking at the best option for "reimporting" my photos from iphoto to photos. Does anyone have some suggestions?

Photos app has approx 10,000 photos. Iphotos has almost 30,000 photos. I found an article about "manually migrating a library into photos" but it sounds like this will create a second library and my assumption would be that the original 10,000 that automatically migrated would be duplicated. I am also worried that if I delete the original 10,000 photos out of the photos app that it will delete them permanently as it sounds like the iphoto and photos app "share" the same photo links.

Greatly appreciate any help/suggestions on the best way to sync the two apps. Thanks in advance!