I think that as long as you accept yourself, and only tell your parents as a matter of polite information instead of a plea for acceptance, it will be fine.
Older parents tend to see the behavior of children as a reflection on themselves, often blame themselves for any deviation from convention.
But if you are calm and reassuring, you can allay their fears.
If they berate you, it could mean they are under the impression that transitioning is some sort of "choice" (showing lack of information on the subject), believing that their disapproval will shame you into getting back in line. If they are religious, they might actually fear for your "soul," so you could view it as a sort of protective behavior.
But if you just laugh and say that it isn't really a choice, then move on to another topic, or just answer their questions, you might educate them enough to relax.
If they become abusive, you can always cheerfully remark that you're sorry they feel that way, and you love them, but of course you must live your life in a way that makes you happy.
As long as you remain cheerful and loving, don't react when they try to push your buttons, and don't act or feel desperate for their approval, it should go well.
You can always cut the visit short by glancing at your watch, saying you have to meet some friends, you love then and will see them later, and then leave.
I'm saying how to do it, not saying that I actually could pull it off without losing my temper.