Syncing photos to iCloud from iTunes/Finder

I always synced my iPhone photos to iTunes, now Finder. I want to put everything in the cloud. When I turn on iCloud for photos it says "Photos synced from Finder will be removed. 8,602 photos will be removed." My photos are currently synced to my MacBook pro. The number of photos my Mac says it has in "photos" is different than what my iPhone says in photos and in recent. How do I get ALL my photos to the cloud without losing 8,602 photos?