Problem with iCloud syncing

Hello !

For some reason I don't understand, my Mac Photos Library stopped syncing with iCloud (as it was no more considered as System Photo Library). My problem : I clicked on the button "Use as System Photo Library" (it is maybe not exactly these therms on the button as I am using the french version of the Mac App) but now, when I try to select iCloud Photo Library, it does not continue the syncing where it stopped but the App want to upload fully the Library without deleting the old one. As my old library (synced) was 120go and my actual 170go (so 50go not synced yet), I do not have enough space on my 200 go iCloud account for both of them... Maybe the best solution would be to erase old library, but I am afraid of losing my local photos by doing that...

I hope I was clear enough...

Do you have a solution to help me solve this problem ?

Thanks a lot ! Martin