Weird. Before I gave my answer, I went to my profile and noticed the edit button next to my username. Now it's gone.
The only difference with before is that I just made a reply, and making a reply might turn the username into read only mode, as it then serves as a way for people to mention you...
So, you can edit your username for as long as it's unlocked. Replying to a post seems to lock it so if you haven't used your account yet, you should be able to use it!
I asked a friend to make an account to test this and use her GMail account to log in. See this screenshot:
The edit button is there until something you do will deactivate it. After that moment, your username will be permanent.