I was thinking I would keep my Photos library referenced by first starting with a referenced Aperture library and then disabling the Photos preference to copy newly imported files into the library. However, there’s a large GOTCHA if you also want to enable iCloud Photo Library. 

After opening a referenced Aperture library in Photos, I told it to become the System Photo Library (in Photos Preferences) which should cause it to merge the iCloud photos with those on the Mac. However I was greeted with the message that referenced images on the Mac WILL NOT UPLOAD TO ICLOUD. They must be consolidated first. Kinda a BIG BUMMER.