Symbols can not be different than they are in the library because kicad does not store symbol information inside the schematic.
Also what you try to do is quite hacky. The best solution really is to copy the original symbol into a project local lib and change it there. (KiCad stores which library a symbol is from inside the schematic. It also caches the symbol in the cache lib so you should be save from future differences in the lib.)
The TlDr version for deriving a symbol from an existing one. The first step would be to make a new symbol library (if you do not yet have a personal library). In the symbol editor file -> new library. I would in this case suggest to store the lib inside the project folder. Also add it to the project library table (more details: Creating a new symbol library and a new symbol in KiCad 5)
Then open the symbol you want to change and use save as to store it into this new library. Then change it however you like. (more details Tutorial: How to make a symbol (KiCad v5.1.x) section Example with separate power uints)