got negw phone. can i erase old from iCloud

iCloud: Remove your device from Find My iPhone - http://support.apple.com/kb/PH2702 "If you’re no longer using an iOS device, an Apple Watch, AirPods, or a Mac, you can remove it from Find My iPhone using Find My iPhone on iCloud.com or your device. When you remove a device, it’s removed from your Find My iPhone Devices list, and if your device has iOS 8 or later, Activation Lock is also turned off."

That depends on what you intend to do with the old phone. If you are going to keep it for a backup, then there is no need to remove it from iCloud. If you plan to sell it or give it away, then it needs to be removed.

Selling/Giving Away Device