I would like to move my photos from my Iphone 6 to Icloud. I have 3500 pictures on my device. When I turn on the Icloud photo library tab I get the following message. Syncing photos via ITunes is not supported when Icloud photo library is turned on. ?

I have 3500 devices on my Iphone 6. I want to download them to ICloud. When I turn on the photo library switch, I get the following message window. PHOTOS AND VIDEOS SYNCED FROM iTunes WILL BE REMOVED. Syncing photos via ITunes is not supported when Icloud photo Library is turned on. 2144 photos and videos will be removed. I don't want to loose any of my photos. From what I have read all photos on the camera roll will be uploaded to ICloud and nothing will be lost. Is that the case. What does the message mean. I wanted to check before I saved to the cloud.