I've been thinking about this a lot recently, as I've got my assessment in just under two weeks.
If I am diagnosed I'm thinking of sending messages to the people that I want to tell, along the lines of:
"I haven't mentioned this before, because it was important to me to have the formal diagnosis and to have come to terms with it myself, but I've been going through the diagnostic process and have been diagnosed as being autistic."
I think I will leave it there for family members that knew me when I was younger and much more 'odd', and for friends I'll continue by saying that I hope it won't change anything, but I just want them to be aware that if I ever do odd things then they aren't intentional, I am aware to some extent, and I hope that they'll understand.
My hope is that it won't change anything, and will simply allow me to feel that I'm being honest with people, but I worry that I'm being optimistic about that.
There's also a hope that, if I did get diagnosed, I would find it easier to bring it in early with people that I meet in future, rather than having to announce it to people that are already close friends...but I guess there is no good time to come out with 'by the way, I'm autistic'. I think, post-diagnosis (?), I will start wearing a bracelet that people might notice.