My company was a little tricky for me.  Just 25 years ago at my company men were required to wear blazers and ties, and women were required to wear skirts or dresses.  And then there were rules like, men could smoke at their desks, and women had to smoke in the bathroom, etc.  So, you get the idea that they were not very forward thinking.  When I started here there were no diversity statements regarding gender identity (shocker!).  So, I messaged one of the HR managers and asked to discuss a personal issue with them.  When I went to her office to meet with her I used all hypothetical statements to find out if the climate here was ok for someone (me) to come out.  I had to have my income (wife, kids, cars, house, etc.), so I was being careful.  She was, of course, thrilled for me (she knew I was referring to myself... lol) and went to the VP of HR, who in turn went to the CEO, to get assurance that i could transition and not lose my job.  Then things cascaded into my favor and I came out post haste!

So, long story short, even for non-progressive companies, just touch base with HR first and see how you think they will react and get their help and guidance.  My HR called my co-workers in one at a time and explained their expectations for treating me professionally and what-not, because I was now outwardly and overtly identifying as female.  Not a single problem ensued. 

Heck, even this year they added trans* surgeries coverage on our healthcare, whereas previously they had been specifically excluded.  Hint: I am the only trans* in the company.