Create new component based on existing one in Eagle 9
It’s fairly simple. You force a symbol duplication and it asks you for a new name. You give it a new name and save it. Then you edit and save the symbol. Ditto the device and device name then delete the old symbol in the new device and install the new symbol. Then you may need to do the same with the footprint(s).

Finally, in the device editor reconcile symbol pin names with footprint numbers and you’re good to go.