What does ‘ Seven years there and he won’t know himself.’ mean?

Question

The school is the finest school and seven years’ studying there should be great for students, but I can’t read it that way for some reason.

“If he wants ter go, a great Muggle like you won’t stop him,” growled Hagrid. “Stop Lily an’ James Potter’s son goin’ ter Hogwarts! Yer mad. His names’s been down ever since he was born. He’s off ter the finest school of witch craft and wizardry in the world. Seven years there and he won’t know himself. He’ll be with youngsters of his own sort, fer a change, an’ he’ll be under the greatest headmaster Hogwarts ever had, Albus Dumbled—“ (Harry Potter 1 [US Version]: p.58) [Bold font is mine]

If seven years there and he still doesn’t understand who he is, the school, I would think, is not so effective. The context is saying, however, the school is very good. Would you guide me to the right meaning?

Answer 1

"He won't know himself" means that he'll have changed so much because he has learned so much skills, that he would practically be unrecognizeable even to himself.

Note the previous sentence, "He's off ter the finest school of witchcraft and wizardry in the world." then, the next statement says that after he's been there seven years, he'll have changed so much, for the better, that he wouldn't be able to tell what he was like before, he wouldn't even recognize himself.

Other examples of usage of "won't know himself" could be:

It's the best beauty parlour in the world. After she's been there, she wouldn't know herself.

The seminar's great. After you attend, you won't know yourself.

Answer 2

They are talking about school for wizardry in a sarcastic way. If I am not wrong then it means "Once you get into the school for wizardry no matter what you will learn it will be not valid in the Muggle world so after spending 7 long years there you wont even know who you are by the end of it"

Correct me if i am wrong ... Thanks