People change the past all the time, for tons of reasons. But specific to this topic, a powerful tool in a therapist's toolbox is 'reframing', where you look at your experiences and find some new perspective. You change your narrative to have more compassion for yourself and the other players (or lay blame on someone more deserving) so that you can move past it now.
One aspect of that can be identifying that you had some special need that wasn't met. Or that your parents or siblings did. Everyone was trying but they weren't equipped to give you what you needed, and it was what it was. Especially for newly diagnosed conditions, your parents 20 years ago wouldn't have had access to the info you have now.
So a diagnosis can in some cases help people reframe all of this stuff.
The label has a net-positive value in that it can allow others, and yourself, to forgive more easily. Now if we could just learn to forgive without the labels, that'd be really valuable.