Jayne, kudos.
The first person I told other than two closest friends was my hairdresser five minutes after I met her. That was a long five minutes, as I had built up the courage before choosing the salon but almost backed down sitting there - maybe next time? And then reminded myself to get this out before they start on my hair, because if it turns out they are the wrong hairdresser (if they respond the wrong way) I still have time to walk away and go find the right one. I had made up my mind beforehand - if they seemed too uncomfortable I would leave a decent tip as I declined and walked off, but if they were rude I'd just bail.
Turns out my hairdresser not only had no problem at all, we have become very good friends.
I know very little about hairdressing but my impression is it involves a mixture of knowledge. They need to understand products, and a surprising amount of science is involved - different hair types, growth patterns, lifestyle (do you swim), humidity. If they are good at it this involves art. With all that, a hair stylist can't aim for good results if you don't tell them what you want. If you are transitioning a little or a lot but haven't outwardly started that's perfectly fine - this might be a long term goal and your hair needs time to change.
My natural hair is curly and a total hassle at an in-between length - think Bozo the Clown. When I told my hair stylist that, she suggested several solutions as my goal was to grow it out - first tame it with a Brazilian Blowout. Later on I went for Japanese Hair Straightening (invented in a country with high humidity), but not too often as that uses some harsh chemicals.
Telling my hairdresser wasn't as critical as telling my doctor(s), but I timed the hair conversation early and glad I did.