Warning – ICloud Photo Library Users: DO NOT Turn Off iCloud ~ Macs Tech Notes
Please file this warning under "unless absolutely necessary." If you use the iCloud Photo Library on Mac, you do not have to log out of iCloud. Do not check the iCloud Photo Library box in either Photos Options in the iCloud Panel in System Preferences or in the iCloud Settings in Photos. Why not? Because, when you enable iCloud or iCloud Photo Library, images will re-upload all your photos, which may take days. (It's not really uploading all of them, but just resyncing will take a long time.)
Worse if you do not have enough space in iCloud for your entire Photos library again, you need to upgrade to a larger plan temporarily resync, and then downgrade to your previous plan. Apple will refund you for the cost of the upgrade, but you must work with support to be refunded. Adam C Angry details hell he experienced on TidBITS .