Well i can kind of relate-
It's hard for me for accepting my new woman body as well, but i really dont wanna put myself in a box
I'm born as a girl, ok, but I still can customise myself as I want, we don't have to put a gender on our soul
I mean, puberty y'know
But anyway, your grandparents had their natural reaction since they don't refer to our "new gender system" (lgbt+ and pronoums), because they're really out of our generation
Of course, you can try to explain to them what lgbt is, and everything that goes with it, but i bet it won't be that easy
But it completely depends on the person !

If I sound offensive, I can delete my comment if you don't like it-
But yea
That's just my opinion 😗