This is a difficult one. Parents can be very unpredictable. When I came out to my mom, I thought she would be supportive, however, in the beginning she was convinced that it was only a "phase". I gave her time, and she eventually came around. I expected my dad on the other hand to reject me, however, he immediately accepted it.
One thing I can pretty much guarantee is that they won't be able to change pronouns right away even if they are supportive. Don't let it get you down, it's often a force of habit rather than an attack. It's taken my nan years to break the female pronoun habit.
I would suggest sitting them down and telling them. They may not like it at first, but if they're open minded people they should come around eventually. In my experience it's better to just get it over with and tell everyone at once.
Good luck to you, I hope it goes well
