icloud photos

I have 3 different iphones on my account and I need to replace one. I just recently started using icloud photos but I found out that I can not delete any photos without removing them from icloud. Can I back up all the devices to icloud, then on the device I need to trade in, set it to not be on icloud anymore so that I can remove all files without losing the photos I had stored in icloud from that device? or do I need to download all the photos from icloud to my PC first so theres a hard copy and then delete them from the phone?