iCloud Photo Libray

I have an iPhone 8 Plus, iOS 11.

I have synced around 900 DSLR photos from my computer via iTunes to my phone; I use them for various needs, some to show clients their photos or mine, etc.

I want to use the option "iCloud Photo Library" sync to sync/backup my live photos that I've taken on my camera phone but I can't as turning this feature on forces me to remove my 900 photographs. Why can't it just ignore those? Why force someone to remove that data?