How can I delete pictures from my iPhone but keep them on my iPad?p

Basically, I have an iphone5s and a iPadAir all my photos are synced through Icloud. I want to purchase a new phone which means trading my old phone and deleting its content. I however would like to keep the pictures saved on myipad but delete them from phone. Every time I attempt to do this the pictures delete from both devices. Does anyone know how to combat this issue?