You have a hard job ahead of you. I hope they react well.
Whether your personal belief includes or does not include the following, I can't say, but I believe that your grandparents may respond at least with a modicum of grace to this:
In religious terms, I would liken gender identity as being a part of the ID of your eternal soul and that Biblically, Eunuchs do exist. Particularly, it is the words in the Bible that Jesus uses that I refer to: "For there are eunuchs who were born that way, and there are eunuchs who have been made eunuchs by others--and there are those who choose to live like eunuchs for the sake of the kingdom of heaven. The one who can accept this should accept it." (Matthew 19:12)
I hope that helps. Good luck.