With the economy the way it is these days nobody is indispensable, for every one of you, there are dozens out there without a job looking, regardless whether you feel you are indispensable just showing up for work as your true self without working with management or an HR department first could get you fired, and if you keep your job when cuts need to be made you could be at the top of their list.
I fully transitioned, I came out to my companies HR department almost 10 months before i told my manager and co workers, like what you are suggesting, i took things slowly, baby steps as it were, it allowed everyone to get used to seeing me.
Paula