I have been out to the department I work in, HR, and a few others I work closely with since the first shot. Thankfully, it's a small department within a huge company and even in the building I work in, 98% of the people don't know my name.
It's been a slow transition, I began binding before T, so the changes have been subtle to those that see me in a hallway etc. Thanks to not knowing my name, those that didn't know me as "old name" will only know me as my true name, no re-introducing necessary.
Since I have been here for years, there was the bathroom dilemma, which I think I solved in a creative way. When I began transition, I began using the bathroom furthest from my little office cave. This gave me the benefit of NOT having those people know who the heck I was. When I switched to the men's room, I began using the one closest to my desk, without incident. But I avoided the issue of someone realizing this person went in one door last week and now a different door. Yes, it was more inconvenient for me in some ways, but easier in others.
The company is amazing with it all though, I have had full backing all the way up the chain since day one, including a note from the CEO and COO. I can not imagine an easier workplace transition that what I experienced.