I would recommend finding a good hair stylist that does both women's and men's hair. Ask for something that is "in between".
My hair stylist was one of the first 5 people I came out to, and I have been going to him for about 3 years. Granted I am located in LA, a magical place for transgendered individuals, but he immediately was on "the page" and gave me exactly what I wanted- an in between haircut that can either be read as male or female.
He added some layers and kept areas long that I will need when I go full time, even pointing them out as he went along with the cut. Similarly to you, I already had long hair before starting HRT.
I can already tell that the fact that my hairstylist knows of my situation will make sure I don't get shorn in a way that will hinder me down the line. I highly highly recommend finding a great stylist, ask for something in between male and female and if you have the confidence explain yourself. They will know what to do, especially if they are the "master" or "lead" stylist at the salon.
By the way, it seems to me that salon stylists LOVE "men" with long hair. Especially straight hair, it's like an open playing field for them. This is a chance for them to be creative AND helpful at the same time in the truest sense. Being honest with them about your situation will get you a really great, awesome cut

The place where I go is called Salon Eleven in downtown Los Angeles, and I've been going to their lead stylist Jared for years. Apparently... he IS trans friendly, so I am happy to recommend him here