Default ICAO codes can not be changed. They can be moved (to your new airport) but not deleted without hacking the bgls, which I do not recommend out of experience. It can create a very glitch ridden and bug infested environment.
I do believe the default name and ICAO are permanent. You can isolate and create a new one with the same ICAO and a new name and see if it works, but I havent tried this so IDK but I dont think it will unless FS overlooks duplicate ICAO codes.
If my thinking is wrong, please correct me. I know a lot but not everything.