domain in domain.fly.dev is the name of your Fly app. I don’t think it is mutable, see: What happens when you remove a dedicated IP? - #2 by kurt. You can, however, deploy the same code to a new Fly app, and name it whatever you like (as long as the name is not taken by others).
I am not familiar with the Ghost blogging platform, so can’t say much what role env vars play here. May be @Curiositry does.