I told my HR back in August that I was transitioning. We haven't established a date yet, but things are definitely noticeable these days (I'm 10 months in on HRT). Even though I do not pass well some things have changed. I have told a few people that I have worked with for a few years now, and they've given me feedback.
Instead of sports bras I found that bra tops work pretty well. You can order them from Victoria's Secret online. They have many different styles so if you're still concerned, you can get one that looks like a tank top.
I'm glad that I had notified my HR back in August because someone that I had confided in told his buddy, and that buddy spread it around the job. Within the day, my name had been linked to the rumor that there was a "->-bleeped-<-" in the hangar. I called HR that day to warn her about what I had heard and she talked to the site supervisor to give him a heads up. They both told me that NO discrimination will be tolerated and not to worry about it. So far it hasn't been a problem. Aside from a couple of people looking at me and trying to figure out if it's true, nothing has been said by my coworkers. However, everyone did have to have sexual harassment and anti-discrimination training this month.

Good luck on your journey.