Transferring to a new iPhone

I got a new iPhone and transferred everything across to the new phone. How do I delete stuff from the old phone so someone else can use it? My photos and everything is backed up to the cloud and it says if you delete photos from the iPhone, it also deletes them from iCloud. Both phones are using the same Apple ID. How do I delete photos from the old phone but keep them on my new iPhone and in iCloud? Also, same with messages, can I delete them from old phone without deleting them from new phone or iCloud?